The structural aspects of short range ordering in the NiMo system is discussed. Particular attention is paid to the relationship between the 1 1 2 O SRO peaks and the occurrence of non-conservative antiphase boundaries within microdomains possessing either the Ni 4Mo or the Ni 2Mo superstructures. The initial stages of the transition from the SRO to the LRO states is believed to involve the transformation of non-conservative APB's to the conservative type.