Photothermal ionization spectroscopy is employed to monitor the residual donor impurities in InP grown by vapor phase epitaxy, and study the effect of adding O2to the carrier gas. The dominant donor species are found to be Si and S, irrespective of the O2addition, and their concentrations are obtained with the aid of Hall measurements. The result shows that the O2addition reduces the incorporation of Si into InP but does not affect the incorporation of S.