The adsorption and thermal reactions of CH₂I₂ and CH₃I, precursors to methyl (CH₃) and methylene (CH₂) surface moieties, have been studied on Ni(110) single-crystal surfaces using temperature-programmed desorption (TPD) and X-ray photoelectron spectroscopy (XPS). Significant self-hydrogenation to methane is observed, as in many analogous systems, but desorption of significant amounts of heavier hydrocarbons, including ethylene, ethane, propylene, propane, and butylene is also observed. A mechanism is operational where the initial hydrogenation of adsorbed methylene to methyl moieties is followed by a rate-limiting methylene insertion step to yield ethyl intermediates. Facile subsequent b-hydride elimination and reductive elimination with coadsorbed hydrogen account for the formation of ethylene and ethane, respectively, while a second and third methylene insertions lead to C₃ and C₄ production. Additional reactions, including some direct methylene and methylene coupling, formaldehyde formation, and free radical ejection, were also seen on oxygen-treated surfaces.