1-Hexene was oligomerized at 200 degrees C and a pressure of 5 MPa in a down-flow fixed-bed tubular reactor filled with beidellite or ultrastable Y zeolite catalysts. Vapor, liquid, and supercritical states of the reacting hydrocarbons in the reactor tube were established by using propane, pentane, octane, and dodecane as solvents. The initial activity, stability with operation time, and selectivity are very dependent on the physical state of the hydrocarbons. Highest activity and stability are reached in the liquid phase using octane and dodecane solvent. The chain length of the solvent has a strong influence on the deactivation of the catalyst, on the oligomerization selectivity and on the formation of Ca saturates and cracked products. (C) 1998 Academic Press.