Thermocouple probing and nuclear magnetic resonance imaging (MRI) method were used to study both heat regimes and liquid distribution under conditions of exothermal hydrogenation reactions proceeding on a catalyst particle and in the trickle bed. The experiments were performed using the model reactions of hydrogenation of a-methylstyrene, octene, and heptene. It was shown that critical phenomena, such as "hot spots" in trickle bed and catalyst particles, multiplicity of steady-state regimes, and hysteresis phenomena, are generated by liquid evaporation and transition of the reaction to the gas-phase mode. The transition is promoted by a number of factors, such as exothermicity of the hydrogenation reaction, presence of dry and partially wetted catalyst particles for liquid superficial velocities lower than 5-6 mm/s, nonuniform distribution of the liquid within the reactor cross section, and phase nonequilibrium in the trickle bed. Multiplicity of the steady-state regimes, hysteresis phenomena, and impact of liquid superficial velocity and catalyst particle size on the onset of critical phenomena were studied experimentally.