Following recent works that report a non linear dependence of the lattice parameter versus composition in some semiconductor alloys the InGaAs/InP system has been investigated. The lattice parameter and the composition of InGaAs/InP lattice matched heterostructures have been independently determined by measuring the high resolution X-ray diffraction profile and the absorption of the X-ray beam diffracted from the InP substrate. In contrast with previous results that stated a linear dependence of the lattice parameter with composition, a 6% larger In content in the InGaAs/InP lattice matched alloy is found. Such result has been confirmed by the analysis of the X-ray fluorescence induced by an electron beam on the layer and on standards made of InAs and GaAs fine ground crystals. The result is in good agreement with the predictions of models based on the elasticity theory applied on a microscopic scale.