[en] The quasi-fission mass distributions have been measured at 365 and 443 MeV lab bombarding energy. Assuming a Markov process, a Fokker-Planck equation has been derived, the coefficients of which have been calculated using a analogy with the classical theory of Brownian motion. These coefficients have been shown to be energy dependent and they have been expressed in terms of a correlation function calculated in an equilibrium situation. This model has been successfully applied to the Cu+Au system at two different bombarding energies. (Auth.)