The orientation of mercaptide (CH₂S-), and the closely similar methoxy (CH₃O-) species on Cu single-crystal surfaces, has been the subject of some controversy. Some near-edge X-ray absorption fine structure results indicate substantially tilted C-S or C-O molecular axes relative to the surface normal, but reflection-adsorption infrared spectroscopy of methoxy on Cu(111), scanned-energy mode photoelectron diffraction of methoxy on Cu(100), and S K-edge surface extended X-ray absorption fine structure of mercaptide on Cu(111) all indicate perpendicular molecular orientations. A technique that provides particularly clear information on molecular orientations is polar angle scanned X-ray photoelectron diffraction, and the authors have applied this method to a study of the molecular orientation of mercaptide on Cu(111). The results show clearly that the mercaptide molecular axis is oriented perpendicular to the surface, a result which is found to be true over a wide range of surface coverages. The origins of the discrepancies between the various techniques are discussed.