There is a growing interest in utilizing in situ-generated dimethyldioxirane (DMDO) as an oxidant for synthetic purposes and bleaching and decontamination applications, but the ability to quantify the organic cyclic peroxide species is often complicated by the presence of other reactive components, peroxymonosulfate and acetone, within the solution matrix. This paper is the first to report the use of a MS method for the quantitation of DMDO from these complex matrices by utilizing an isothermal 30 degrees C GC program in conjunction with selected ion monitoring (SIM). The volatile organic species is sampled from the headspace of closed batch system vials and quantified by measuring the abundance of m/z 74. The method achieves a practical quantitation limit (PQL) for DMDO of 0.033 mM, and methyl acetate is identified as a minor decomposition product from the aqueous sample matrix, contributing 9% towards the overall DMDO measurements. The spectroscopic method makes use of common analytical instrumentation and is capable of measuring other in situ-generated dioxiranes, such as those generated from 2-butanone and [2H6]acetone.