Phenylacetoxycarbene, phenyl(pivaloyloxy)carbene, and phenyl(benzoyloxy)carbene, photolytically generated from diazirine precursors in pentane at 25 degrees C, efficiently rearranged by 1,2-acyl migrations to give high yields of the appropriate 1,2-diketones. The kinetics of these rearrangements were determined by laser flash photolysis. Substituent effects on the acyl migrations and ab initio electronic structure calculations on ground state carbenes and transition states were employed to analyze the rearrangement mechanism. Additions of phenylacetoxycarbene to alkenes proceeded in good yields, in lieu of the 1,2-acyl shift; absolute rate constants were obtained for these reactions of the ambiphilic carbene. (Phenoxymethyl)acetoxycarbene gave only a 1,2-H shift; the potentially competitive 1,2-acetyl migration was suppressed.