The mechanism of aggregation of bovine serum albumin (BSA) by poly(allylamine) hydrochloride (PAH) is investigated as a function of the mixing ratio r defined as the ratio of the number of BSA molecules and PAH chains present in the solution. under pH conditions of strong binding between the two partners. It is found that as r increases the turbidity first increases, passes through a maximum at a value r(max) before decreasing again. For small and large values of r, one forms small aggregates in the 10 nanometer size range, whereas at r(max), the size of the aggregates becomes of the order of micrometers. The structure of the aggregates appears to be independent of the history of the systems but depends only on the value of r despite the strong BSA/PAH binding. The desaggregation of the large aggregates formed at r(max) by the addition of BSA or PAH is shown to be an isenthalpic process and is thus entirely entropically driven. Moreover, we prove that r(max) corresponds to the state of the system where both the PAH chains and the BSA molecules interact one with each other, both with their maximum number of interaction points per molecule. This explains the independence of r(max) on the BSA or PAH concentration in the solution and why it varies linearly with the molecular weight of the polyelectrolyte. Moreover, we show that at r(max) all the BSA and PAH molecules present in the solution are involved in the aggregates. At small (respectively large) values of r, the aggregates appear positively (respectively negatively) charged, corresponding to a charge excess at the surface of the aggregates. Finally, it is found that the protein/polyelectrolyte interaction is endothermic, indicating that the BSA/PAH binding must thus be entropically driven. The binding enthalpy of BSA molecules with PAH chains for r < r(max) is of the order of 400 kJ.mol(-1) of BSA molecules for solutions containing less than 0.1 M of NaCl. The effect of the salt concentration of the solution on the binding process is also briefly discussed.