2-Acetoxy-2-methoxy-5,5-dimethyl-Δ3-1,3,4-oxadiazoline undergoes two competitive 1,3-dipolar cycloreversions at 110 °C. It loses N2, presumably to afford a short-lived carbonyl ylide that fragments to acetone and acetoxy(methoxy)carbene. It also forms 2-diazopropane and the appropriate mixed anhydride. It is the only currently known source of acetoxy(methoxy)carbene. Key words: acetoxy(methoxy)carbene, 2-diazopropane, 1,3-dipolar cycloreversion.