The chemical composition of multicomponent alloy surfaces may exhibit significant deviations from the bulk composition due to thermally activated segregation and cosegregation processes. In many systems cosegregation phenomena result in the formation of two-dimensional surface compounds such as CrN on Fe-15% Cr-N(100), VC on Fe-3% V-C(100) and on Fe-3% Si-0.04% V-C(100), and TiC on Fe-6% Al-0.5% Ti-C(100). These surface compounds are epitaxially stabilized on the bcc(100) alloy surfaces, their thicknesses are about one or two atomic layers. In the present work we report results of a Monte Carlo study on the cosegregation-induced formation of surface compounds on bcc(100) surfaces. The simulations are performed utilizing a three-dimensional lattice gas model with two free (100) surfaces and periodic boundary conditions in x and y directions. It is assumed that the lattice consists of two types of lattice sites M and X. The metal sites M form a body-centered cubic (bcc) lattice, whose quasi-octahedral interstices constitute the nonmetal sublattice. The M sites are accessible to either M(A) or M(B) atoms, while the nonmetal sites either are occupied by X atoms or remain empty. Pairwise repulsive nearest and attractive next nearest neighbor interactions between M(A)-X and M(B)-X atoms are considered as well as up to fourth nearest neighbor X-X repulsions. The simulations indicate that cosegregation-induced formation of the surface compound M(B)X is basically due to preferential next nearest M(B)-X neighbor attractions. With an increase of the strength of the preferential next nearest M(B)-X attractions, surface compound formation is accompanied by a first-order phase transition. Depending on the relative magnitude of the nearest neighbor M-X repulsions, we observe a strong M(B) subsurface enrichment, which has been verified by XPD for the CrN surface compound on Fe-15% Cr-N(100). For repulsive fourth nearest X-X neighbor interactions, our lattice gas model shows c(2x2) ordering of the X atoms on the bcc(100) surface.