The molecular beam epitaxial (MBE) growth of Al(0.48)In(0.52)As lattice matched to InP (001) and Al(0.70)In(0.30)As strained on GaAs (001) has been studied through reflection high energy electron diffraction intensity oscillations. The reported experimental results illustrate the influence of the substrate induced strain and the partial pressures of the elements III and V on the In incorporation coefficient, α In, in MBE grown AlInAs alloys. At a given temperature, α In is shown to decrease by increasing the substrate induced strain and by decreasing the partial pressures of the elements III and V. In order to explain these results, a thermodynamic model is proposed, taking into account the additive strain Gibbs free energy, function of the elastic coefficients of the materials, and the excess Gibbs free energy of mixing of the regular ternary solid solution.