Propofol (2,6-diisopropyl phenol) is a widely used intravenous anesthetic. To define its pharmacokinetics and pharmacodynamics, methods for its quantitation in biological matrixes have been developed, but its pattern of mass spectral fragmentation is unknown. We found that fragmentation of the [M Ϫ H] Ϫ ion (m/z 177) of propofol in both APCI MS/MS and ESI MS/MS involves the stepwise loss of a methyl radical and a hydrogen radical from one isopropyl side chain to give the most intense product ion, [M ϪH Ϫ CH 4 ] Ϫ , at m/z 161. This two-step process is also the preferred mode of fragmentation for similar branched alkyl substituted phenols. This mode of fragmentation of the [M Ϫ H] Ϫ ion is supported by three independent lines of evidence: (1) Ϫ product ion. Phenols with a single straight chain alkyl substituent, in contrast, undergo  elimination of the alkyl radical irrespective of the length of the alkyl chain, yielding the most intense product ion at m/z 106. This product ion represents a special case of a stable intermediary radical for the two-step process described for branched side chains, because further elimination of a hydrogen radical from the  carbon is not possible. as a supplement to regional anesthesia and in critically ill patients confined to intensive care units. The patient loses consciousness 30 to 50 s after receiving the drug and remains asleep for about 4 to 6 min [1,2]. Even though a single dose of propofol has a short duration of action and produces fast recovery from anesthesia, propofol is not metabolized in a few minutes. It is present in blood, adipose tissues, liver, and brain for a few hours, its concentration decreasing with time [4].