In the Co–Pt system, a cooling experiment can drive a sample ordered in the cubic L1 2 structure inside the two-phase region involving L1 2 and the tetragonal L1 0 structure. We had shown previously (Phys. Rev. B. 61(5) (2000) 3317) that the first step of the microstructural development is the decoration of the antiphase boundaries (APB's) in the L1 2 structure by a thin L1 0 layer. Using transmission electron microscopy observations, we present here the later stages of the microstructural evolution, i.e. when the cooling procedure induces a larger amount of L1 0 structure in the sample. Depending on the aging temperature, the final micro-structure consists of large L1 2 domains separated by either thick platelets of the L1 0 structure, or by a saw-tooth like morphology. We explain these microstructural developments by a theoretical analysis of the accommodation of the elastic stresses generated by the difference in symmetry and lattice parameters between the L1 2 and L1 0 structures.  2001 Acta Materialia Inc. Published by Elsevier Science Ltd. All rights reserved.