Disproportionation of the liquid mixed anhydride dimethacrylic sebacic anhydride (m-SA) to methacrylic anhydride and polyanhydride oligomers, as well as vinylic polymerization, can occur on workup and storage at room temperature. Dimethacrylic 1,3-bis(p-carboxyphenoxy) propane, being a solid, can also disproportionate, but the propensity to do so is lower than for m-SA. These events can be suppressed with free-radical inhibitors, and the inhibitor 2 + 3-t-butyl-4-methoxyphenol is a more effective stabilizer than 4-methoxyphenol at 20.degreesC.