High-level single-reference calculations reveal that trimethylphosphine-mediated reductive dimerization of properly substituted (e.g., CF3) ketones proceeds via initial formation of an oxaphosphirane intermediate, with the oxygen atom occupying an equatorial position at phosphorus. In the "oxirane route", this oxaphosphirane intermediate loses a trimethylphosphine oxide unit, thus behaving as a carbene transfer agent toward a second carbonyl molecule and giving rise to a carbonyl ylide that cyclizates to the corresponding oxirane. This in turn transfers the carbene unit to a second phosphine molecule, with loss of acetone, affording a phosphorane. The latter undergoes typical Wittig reaction to the final homocoupling product through the oxaphosphetane intermediate. The alternative direct conversion of oxaphosphirane into phosphorane constitutes the lowest energy path as it skips the highest barrier oxirane → phosphorane conversion. The oxirane route is favored by the use of polar solvents and electron deficient carbonyl components. The lowest barrier most exergonic process from oxaphosphirane is the pericyclic cycloaddition of the acetone C═O bond along the endocyclic P-C bond, furnishing the stable 1,3,2-dioxaphospholane product.