The entropy evolution in heavy ion collisions is investigated by means of a cascade model study. The method for calculating the entropy is based on a smoothing over the momentum space by introducing a local temperature field. It is shown that the resulting specific entropy is rather sensitive to the proper choice of the phase space subdivision at the disassembly stage of the collision. Compared to recent results for specific entropy values inferred from the composite particle yield of 4..pi.. measurements, it is found that the cascade calculations do not favor other model treatments, but as a trend we obtain smaller entropy values than following from conventional considerations within equilibrium statistics.