By means of Monte Carlo simulation techniques the relaxation of a packet of charge carriers migrating in a Gaussian density of states of width σ has been studied. The relaxation time determines the transition from dispersive to non-dispersive transport seen in a time-of-flight signal. For a 10 μm thick sample under a bias field of 105V cm−1the critical disorder parameter σ/kT turns out to be 4·4. Consistency with experimental results from polymers is found.