The chemisorption of ethylene and halogen-substituted alkenes has been studied by X-ray (Al K$\alpha $) and ultraviolet (HeI and HeII) photoelectron spectroscopy. Initial dissociation of the vinyl halides at submonolayer coverage is followed by an associative regime at higher coverages; a mechanism for the surface reaction, involving oxidative addition (metallation) of carbon-hydrogen bonds and subsequent dehydrohalogenation, is proposed. The reactivity of ethylenic carbon-hydrogen bonds is shown to depend strongly on adjacent halogen substitution. An intermolecular (Langmuir-Hinshelwood) mechanism is demonstrated for hydrogen halide elimination from mixed adlayer surfaces of chlorotrifluoroethene and ethene or other alkenes with reactive carbon-hydrogen bonds. A statistical model in which dissociation requires the presence of two adjacent vacant coordination sites accounts semi-quantitatively for the dissociative-associative pattern of sorption as a function of coverage, for the differing reactivities of the alkenes at the Pt(100) and Pt(111) substrates and for the associative sorption of the vinyl halides which follows preadsorption of fractional monolayers of carbon monoxide.