Abstract The adsorption of oxygen and hydrogen on a Pt(111) surface has been studied with phase-sensitive Second-Harmonic Generation (SHG) signal detection. The SHG-signal change measured with p-in, p-out polarization during the adsorption of oxygen and hydrogen was found to be different in amplitude and phase for the two adsorbates. At the wavelength used (1064 nm), only a localized interaction between adsorbate and substrate is seen, leading to a linear dependence of the susceptibility on the coverage. Sticking coefficients, , and their coverage dependence were determined. For hydrogen, a linear decrease in with coverage was found; the initial sticking coefficient being at a temperature of K. For oxygen, shows a quadratic decrease with coverage, strongly dependent on temperature, with at K. A method based on these results is proposed, which would allow the determination of adsorbate coverages of coadsorption systems with SHG using phase-sensitive signal detection. PACS68.35; 82.20; 42.65