Monte Carlo calculations and numerical integrations are used to study molecular weight distribution (MWD) in carbocationic polymerization and to illustrate the effect of equilibrium constants, rate constants, and concentration of common ions on MW and MWD. It is demonstrated that bimodal molecular weight distribution is obtained when exchange between ions and ion pairs of similar reactivities is slow and their lifetimes are sufficiently different. Bimodal MWD is converted to narrow (Poisson) distribution in the presence of salts with common ions; molecular weights in these systems grow linearly with conversion like in living polymerization.