Curves are presented for the determination of the proportional contents of ingredients in InxAl1−xP, InxGa1−xP, and AlxGa1−xP from the relative intensity of x-ray fluorescence of In, P, and Al in these materials. The curves apply for the case where fluorescence is induced in the ternary compounds by the 5.9-keV x rays from an 55Fe radioactive source. Each curve has to be calibrated at one point with pure InP or AlP to normalize the system for detector efficiency and for inaccuracies in the cross sections used in the calculations. The accuracy of the results depend on x. Assuming inaccuracies of ±10% in all the coefficients used, the worst deviation in x is ±0.04 at x=0.50. Experimental verification is presented for the case of InxGa1−xP .