The role of thiol/disulphide exchanges during acid gelation of preheated milk was studied with milk samples with or without N-ethylmaleimide (NEM), a thiol-blocking agent, and acidified to pH 4 by the addition of glucono-delta-lactone at 20 °C. Active or total thiol groups, particle size with light scattering measurements in a dissociating solvent or by SDS-agarose electrophoresis were determined on acidified milk samples. Diffusing wave spectroscopy and rheology in low strain were applied during acidification of sample, while rheology in large strain was applied on final acid gels. The only effect of the presence of NEM was a reduced firmness of acid gels as measured at large strain and a reduced tendency to form large aggregates at pH<5.5. In conclusions, thiol/disulphide exchanges during acidification of milk played only a minor role in the building of acid gel networks from heated milk.