A theoretical model is developed which predicts the compositional dependence of the thermodynamic properties of intermetallic phases exhibiting the triple-defect B2 structure. This type of structure is typically formed between elements of Group VIIIB ( A metal) and elements of Group IIIA ( B metal). For the thermodynamic activity of the components A and B, the following simple expressions are obtained: ( a A a A,0 ) = ( α z ) and ( a B a{B,0} ) = ( z α ) , where z is the vacancy concentration at any composition and α, the disorder parameter, is the vacancy concentration at the stoichiometric composition. For the partial enthalpy, the model predicts a step function, with essentially constant values of the partial enthalpy, below and above the stoichiometric composition. The defect concentration as a function of composition is given by the equation: z 2( z − 2 χ) = α 3, where z is the vacancy concentration and (z − 2χ) 2 is the concentration of anti-structure atoms. The parameter, χ = x B − 0.5, expresses the deviation from stoichiometry. The model is applied to the following systems for which thermodynamic data are available: FeAl, CoAl, CoGa, NiAl, NiGa, RuAl, RuGa, RhAl, PdAl, PdIn, IrAl. It is found in general that the theoretical expressions for activity, partial enthalpy and vacancy concentration agree well with the experimental data. The values of the disorder parameter, α, range from 1.3 × 10 −4 for CoAl to 4 × 10 −2 for CoGa. An equation for the temperature dependence of the disorder parameter is given and a relationship between the enthalpy of formation and the disorder parameter is derived.