A study of the order-disorder L1₂ to A1 transformation near a congruent point of the Co-Pt phase diagram is presented. This work was performed by combining in situ transmission electron microscopy observations, resistivity measurements carried out in conditions near thermodynamic equilibrium and high-resolution imaging on quenched samples. Resistivity measurements display three characteristic temperatures: 994 K, 1020 K and 1032 K. The evolution of the microstructure with temperature at different heating rates was followed in dark field images, until the order-disorder temperature was reached. It was found that at temperatures above 994 K ( approximately=40 K below T_c), the antiphase boundaries (APB) undergo a structural change; the phenomenon becomes more pronounced as the temperature increases. High-resolution images of modified APB show thin layers of disordered phase in the core of these APB. This can be described as a wetting of the antiphase boundaries by the disordered phase. Above 1020 K, up to 1032 K, a second mechanism is superimposed; the disordering occurs through a nucleation, growth and coarsening of disordered regions inside the ordered domains. This nucleation and growth process starts at the beginning of the two-phase region (1020 K). These results are in excellent agreement with those from cluster variation method calculations performed by Kikuchi and Cahn (1979) and Finel and co-workers (1990).