Oxygen from air improves the eerie methane sulfonate-mediated electrochemical oxidation of 2,4-pentanedione, 1,3-cyclohexanedione, 1,3-diphenyl-1,3-propanedione, diethyl malonate, and ethyl acetoacetate, giving fragmentation products. An increase in the yield and decrease in the time required for these transformations with no changes in the products or in the electron stoichiometry allow us to propose a mechanism where the oxygen oxidizes an intermediate complex formed between the substrate and the mediator, in a fast step. A dioxygen radical intermediate formed in the process is anodically reduced.