The mass spectra of the methyl ester of the new unsaturated pentacyclic triterpenoid ifflaionic acid (urs-12-en-3-on-30-oic acid) and some of its chemical derivatives are presented. These may be used to assign position C-20 to the carbomethoxy and C-3 or C-7 to the keto group. Mechanistic proposals for, and the effect of functional groups on, the main fragmentation of the molecular ions (cleavage through ring C) are discussed.