Bimodality has been observed on the asymmetry between the charges of the two heaviest products resulting from the decay of the QP released in peripheral Xe+Sn and Au+Au collisions from 60 to 100 MeV/u. Bimodality is observed for intermediate impact parameters for which the quasi-projectile is clearly identified. The two event families correspond to different excitation energies as it is expected in a first order phase transition picture. It seems that the relevant parameter which governs the phenomenon is the dissipated energy per nucleon. It is also shown that it is possible to correlate the bimodality signal with the negative heat capacity and the Delta scaling. All together, this set of data is coherent with a liquid-gas phase transition of finite pieces of nuclear matter.