The variation of the lattice parameter ($a_{0}$) with composition was determined for series of $(Cr, Mn)_{23}(B,C)_{6}$ solid solutions. With a constant ratio of non-metal atoms, variation of $a_{0}$ presents a single point for the atomic Mn : Cr ratio 65 : 35. Therefore Cr—Mn substitution is of an orderly type and chromium preferentially occupies the (f) position in the lattice. Boron-carbon substitution is of the statistical type and involves position (c) only: the lattice parameter increases linearly as boron content increases. Chromium-manganese substitution is complete at 1123 K for all terms containing boron up to 27% atomic. The cubic phase area decreases as temperature increases and terms with higher chromium and boron contents are systematically more stable. All terms of solid solution melt incrongruently with initial liquid formation temperature less then 1850 K (incongruent $Cr_{23}C_{6}$ melting point).