Oxidative addition (OA) is an important elementary step in chemistry, but it has been studied mainly in the context of transition-metal-catalyzed reactions and mainly with carbon-X substrates (X = halogen, H). Reports of main-group metal compounds undergoing OA are rare by comparison, and those involving phosphorus-halogen substrates are rarer still. Acyclic and cyclic diazagermylenes and -stannylenes react with chloro(hydrocarbyl)phosphines with the intermediacy of oxidative addition products. Stannylenes react faster than germylenes, and these reactions are first-order in both reactants and slowed by steric bulk. Kinetic data and the structures of intermediates and products had suggested an adduct/insertion mechanism for these reactions. To gain further insight into these transformations, the work presented herein was extended to chloro(hydrocarbyl)phosphines with varying organic substituents. These studies confirmed prior conclusions concerning the rate-diminishing effect of steric bulk, and the rate dependence on leaving groups also seems to suggest adduct/insertion or SN2 mechanisms. Importantly, these new data now also point to associative decomposition pathways. In the course of the investigation, it was discovered that aliphatic chloro(amino)phosphines react differently with the carbene analogues, giving oxidative addition products for germylenes but metathesis reactions for stannylenes.