Abstract Axial temperature profiles were measured in a bed of porous catalyst particles through which liquid trickled. The hydrogenation of cyclohexene in cyclohexane on a palladium carrier-type catalyst was used as a model reaction. The measurements were carried out in a laboratory trickle bed reactor. Special attention was paid to the vaporization of the liquid phase in the system which results from the heat of reaction. The effects of hydrogen flow rate, concentration of the substrate in the reaction mixture and feed temperature on the temperature profiles in the bed were studied. The vaporization of the liquid phase at some point in the bed is apparent from a sharp increase in temperature at this point (the formation of a hot spot in the bed) as a result of the great difference in effectiveness of the catalyst operating in the liquid and gaseous phases. In the liquid phase, the mass transfer resistance in a catalyst pellet is much higher than that in the gaseous phase. Disappearance of this resistance causes an increase in the reaction rate and overheating of the catalyst and reaction mixture.