Abstract : The availlability of beams of very heavy ions at the GANIL accelerator facility has enabled us to undertake the study of the 208Pb + 197Au reaction at 29 MeV/u. The investigation of ths heavy symmetric system has been carried out through the study of the associated neutron multiplicity. In the first part of this thesis (composed of four sections) we described the experimental techniques and data processing, with particular emphasis on the neutron detector ORION and the hodoscope used to detect the charged reaction products at forward angles. In the second part we present the experimental results, in which we show that the neutron multiplicity is a good measure of the violence of the collision and a good way to characterize the different modes of the reaction. In the third part we characterize the fission of the quasi-projectile and we show that this occurs for peripheral collisions. Using simulations we determine the minimal values of the angular momentum transfered to he quasi-projectile. In the fourth part, we describe the results of dynamical calculations made using the Landau-Vlasov equation which show the importance of angular momentum. In this work we demonstrate that, at 29 MeV/u, the Pb+Au collision in still governed by Deep Inelastic reactions in which angular momentum in the exit channel plays an important role. A complete dissambly of the system in intermediate mass fragments is observed for the most central collisions.