Results from our recent simultaneous measurement of all three component activities by Knudsen effusion mass spectrometry, together with results available in the literature from vacancy concentration and calorimetric measurements, have been used in a thermodynamic modelling study. The results of this modelling for the binary B2-AlFe and B2-AlNi phases are presented in this communication. A four-sublattice cluster energy model in the point approximation has been used for the configurational contributions to the Gibbs energy. Excitational free energies are incorporated into the configurational energy and non-configurational contributions are considered to be decoupled from the configurational. The modelling has been found to lead to good agreement between calculated and experimental results for both the thermodynamic properties and the vacancy concentrations. It is clear that B2-AlFe is an antisite defect compound at both the lowest and highest temperatures with vacancy defects only predominating, and to only a small degree, at stoichiometry in an intermediate temperature range. In the case of B2-AlNi, however, vacancy defects predominate at stoichiometry over the whole temperature range so that it can be correctly described as a neartriple defect compound.

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