A microscopic model for the strain-controlled atomic ordering of interstitial impurity atoms and for the strain-induced decomposition reaction in their solid solutions based on host crystals with a polyatomic basis is used. It takes into account the 'blocking' and static-displacements' effects in interactions between impurity atoms arranged on different-type interstices, in order to explain the salient features of distribution of the diffuse-scattering intensity in reciprocal space for such interstitial solid solutions based on some hexagonal close-packed (h.c.p.) transition and rare-earth metals.