Systems and methods are provided for detecting abnormal temperature conditions within a fixed bed reactor. In a fixed bed reactor, a hydrocarbon (or hydrocarbon-like) feedstock can be exposed to one or more types of catalyst particles at elevated temperatures and/or pressures. In addition to the one or more types of catalyst particles, a plurality of temperature-sensor structures can be included in a catalyst bed and/or a coating including one or more temperature-sensor structures can be provided on an interior wall or other interior surface in the reactor. The temperature-sensor structures can have a threshold temperature at which the temperature-sensor structure changes to allow introduction and/or release of a detectable substance.