The alkylation of benzene with ethylene over beta zeolite to produce ethylbenzene is conducted in all five near-critical regions: supercritical fluid, low-pressure vapor, vapor-liquid two-phase, high-pressure liquid, and low-pressure liquid. The operating conditions vary from 5.4 to 7.4 MPa at an ethylene mole fraction of up to 0.3. A differential reactor is applied to study the reaction behavior, primarily the reaction rates under different conditions. When the operating pressure remains unchanged the reaction rate sometimes undergoes two rate peaks with an increase in the ethylene fraction. Correlations between the reaction rate and the pressure show that the maximum rates can be found under pressures near the critical pressure. The rate "valley" corresponds to the liquid-vapor two-phase region. This might be explained by the fact that, in the two-phase region, the repeated phase shifts can cause some harmful effects on the catalyst.