In a liquid full plug flow reactor during lower olefin dimerization, the assumption of constant density is not valid-the volume of a plug changes as it proceeds along the reactor. The observed kinetics depend on the density variation in the reactor as the conversion proceeds towards a distribution of oligomers-and this in turn is a function of the reactor conditions for the olefin dimerization. The net effect of considering density changes in elucidating global kinetics is to increase the effective reaction order; this is a function of the degree of oligomerization. The effects are demonstrated with a simple analytical model based around varying conditions during dimerization of 1-butene over an amorphous catalyst.