The complex formation model of Bhatia and Hargrove (BH) to explain the thermodynamic properties of compound forming A-B alloys is reformulated using a quasi-lattice picture. The formulation is explicitly carried out to an approximation where the pseudo-ternary alloy (of A and B atoms and AμBνcomplexes (μ, ν small integers)), envisaged in BH, is treated in the quasi-chemical approximation. It is advantageous over BH in two major respects: Firstly, it gives useful insight into the two approximations used in BH, since these follow from it by simply going to a lower (zeroth) approximation and setting the coordination numberzof the alloy to bez= 2 andz= ∞. Secondly, unlike the BH approach, it provides also an expression for the short range order parameter α1for nearest neighbours. The effect of varyingzin the formulae is examined and a brief discussion of α1is given.