AbstractA re‐investigation of the photolysis of acyl phosphonic acid dimethyl‐ and diethyl esters revealed that commonlyα‐scission occurs although the quantum yieldϕ(α)depends on the chemical nature of the acyl group:ϕ(α)= 0.2‐0.3 in the cases of pivaloyl and substituted benzoyl phosphonates andϕ(α)=0.03 in the case of benzoyl phosphonic acid diethyl ester (BPDE). The major route of deactivation of electronically excited phosphonates involves intersystem crossing (ISC) to the triplet manifold. ISC was studied by triplet quenching using naphthalene. Quantum yields of triplet formationϕ(T) = 0.5‐0.6 were found in all cases except BPDE (ϕ(T) = 0.9). The reaction mechanism of a typical phosphonate (2,6‐dichlorobenzoyl phosphonic acid dimethyl ester, DCPME) is discussed on the bas