In the present work the authors focus on body-centered cubic (bcc) metals such as [alpha]-Fe, [beta]-Ti and [beta]-Zr with interstitially dissolved nonmetals like carbon, nitrogen and oxygen. The solubilities of these elements in [alpha]-Fe is generally relatively small, while the solubilities in the high temperature modifications [beta]-Ti and [beta]-Zr are about 5--10 atom percent. On the other hand, the solubilities of interstitial in face-centered cubic (fcc) [gamma]-Fe or hexagonal closed packed (hcp) [alpha]-Ti and [alpha]-Zr are significantly larger. Upon quenching, homogeneous austenitic solutions of [alpha]-Fe with high carbon or nitrogen contents displacive phase transformations to nitrogen and carbon martensites with body-centered tetragonal (bct) structures are observed. Despite the fact that these martensites are thermodynamically instable with respect to the formation of [alpha]-Fe and carbides or nitrides, martensitic phase transformations play an important role in industrial processes such as steel hardening. Upon aging, ordering of the interstitial is observed in some martensites, e.g. in Fe-N martensites leading to Fe[sub 16]N[sub 2]. In the present work a three-dimensional lattice gas model for the description of bcc and bct gas-metal solutions is introduced. Pairwise interactions between adjacent interstitial are considered. Characteristic properties of such lattice gas models are calculated utilizing the Monte Carlomore » method. It will be demonstrated below that ordering of the interstitial is the rule rather than the exception, at least at low temperatures. The ordered lattice gas phases observed are identified and will be discussed in detail.« less