Angle-resolved X-ray photoelectron spectroscopy (ARXPS) was used to characterize the bonding of alkanethiols to n-InP surfaces and to measure the monolayer thickness. A careful comparison of the angle-dependent spectra for three different sample preparations (oxidized InP(100), HF-etched InP(100), and HF-etched InP(100) with a self-assembled monolayer of C12H25SH) shows that the sulfur binds to In atoms on the surface. Comparison of the angle-dependence of the intensity ratios for the In 3d(5/2) core level of InP with the self-assembled monolayer (SAM) to that of sputtered InP provides a thickness for the SAM. of 14 +/- 4 Angstrom, corresponding to alkane chains at a tilt angle of 44 +/- 16 degrees from the surface normal.