diff --git a/esphome/components/emontx/__init__.py b/esphome/components/emontx/__init__.py
index 0262f8681f1..971f9769246 100644
--- a/esphome/components/emontx/__init__.py
+++ b/esphome/components/emontx/__init__.py
@@ -44,13 +44,29 @@ def _get_data() -> EmonTxData:
     return CORE.data[DOMAIN]
 
 
+def _synchronous_automation(value: ConfigType) -> ConfigType:
+    """Reject deferring actions: the received line and parsed JSON are only valid while the
+    automation runs, and DelayAction and friends keep the trigger arguments for later."""
+    if automation.has_non_synchronous_actions(value):
+        raise cv.Invalid(
+            "Deferring actions (delay, wait_until, script.wait, ...) are not allowed in on_json and "
+            "on_data: the received data is only valid while the automation runs. Copy what you need "
+            "into globals first, then defer in a separate script or automation."
+        )
+    return value
+
+
+def _synchronous_automation_schema() -> cv.All:
+    return cv.All(automation.validate_automation({}), _synchronous_automation)
+
+
 # Main configuration schema
 CONFIG_SCHEMA = (
     cv.Schema(
         {
             cv.GenerateID(): cv.declare_id(EmonTx),
-            cv.Optional(CONF_ON_JSON): automation.validate_automation({}),
-            cv.Optional(CONF_ON_DATA): automation.validate_automation({}),
+            cv.Optional(CONF_ON_JSON): _synchronous_automation_schema(),
+            cv.Optional(CONF_ON_DATA): _synchronous_automation_schema(),
         }
     )
     .extend(cv.COMPONENT_SCHEMA)
@@ -104,10 +120,10 @@ _CALLBACK_AUTOMATIONS = (
     automation.CallbackAutomation(
         CONF_ON_JSON,
         "add_on_json_callback",
-        [(cg.JsonObject, "json"), (cg.std_string, "raw_json")],
+        [(cg.JsonObject, "json"), (cg.StringRef, "raw_json")],
     ),
     automation.CallbackAutomation(
-        CONF_ON_DATA, "add_on_data_callback", [(cg.std_string, "data")]
+        CONF_ON_DATA, "add_on_data_callback", [(cg.StringRef, "data")]
     ),
 )
 
diff --git a/tests/component_tests/emontx/test_synchronous_automations.py b/tests/component_tests/emontx/test_synchronous_automations.py
new file mode 100644
index 00000000000..c190c14ab19
--- /dev/null
+++ b/tests/component_tests/emontx/test_synchronous_automations.py
@@ -0,0 +1,54 @@
+"""Tests that emontx on_json/on_data automations reject deferring actions.
+
+The triggers pass views into the receive buffer and the parsed JSON document, which are only valid
+while the automation runs, so the deferring-actions guard is a safety property.
+"""
+
+from collections.abc import Callable
+
+import pytest
+
+from esphome import config_validation as cv
+from esphome.components.emontx import CONF_ON_JSON, CONFIG_SCHEMA
+from esphome.const import CONF_ON_DATA
+from esphome.core import Lambda
+from esphome.types import ConfigType
+
+TRIGGER_KEYS = [CONF_ON_JSON, CONF_ON_DATA]
+
+DEFERRING_ACTION = {"delay": "1s"}
+SYNCHRONOUS_ACTION = {"lambda": Lambda('ESP_LOGD("test", "ran");')}
+TRUE_CONDITION = {"lambda": Lambda("return true;")}
+
+DEFER_MESSAGE = "Deferring actions"
+
+
+def _trigger_schema(trigger_key: str) -> Callable[[list[ConfigType]], list[ConfigType]]:
+    """The validator CONFIG_SCHEMA applies to this trigger key."""
+    return CONFIG_SCHEMA.schema[cv.Optional(trigger_key)]
+
+
+@pytest.mark.parametrize("trigger_key", TRIGGER_KEYS)
+def test_synchronous_automation_accepted(trigger_key: str) -> None:
+    _trigger_schema(trigger_key)([{"then": [SYNCHRONOUS_ACTION]}])
+
+
+@pytest.mark.parametrize("trigger_key", TRIGGER_KEYS)
+def test_deferring_action_rejected(trigger_key: str) -> None:
+    with pytest.raises(cv.Invalid, match=DEFER_MESSAGE):
+        _trigger_schema(trigger_key)([{"then": [SYNCHRONOUS_ACTION, DEFERRING_ACTION]}])
+
+
+@pytest.mark.parametrize("trigger_key", TRIGGER_KEYS)
+def test_nested_deferring_action_rejected(trigger_key: str) -> None:
+    actions = [{"if": {"condition": TRUE_CONDITION, "then": [DEFERRING_ACTION]}}]
+    with pytest.raises(cv.Invalid, match=DEFER_MESSAGE):
+        _trigger_schema(trigger_key)([{"then": actions}])
+
+
+@pytest.mark.parametrize("trigger_key", TRIGGER_KEYS)
+def test_deferring_action_in_second_automation_rejected(trigger_key: str) -> None:
+    with pytest.raises(cv.Invalid, match=DEFER_MESSAGE):
+        _trigger_schema(trigger_key)(
+            [{"then": [SYNCHRONOUS_ACTION]}, {"then": [DEFERRING_ACTION]}]
+        )