Two membrane inlet mass spectrometric (MIMS) methods for determining phenolic compounds in water are described and compared, namely direct analysis and analysis after acetylation of the phenolic compounds. Direct analysis of phenolic compounds in water is a very simple and rapid method and detection limits are relatively low (from 30 μg 1−1 for phenol to 1000 μg 1−1 for 4-nitrophenol). Analysis of phenolic compounds after aqueous acetylation is also a very simple and rapid method, and the detection limits are even two orders of magnitude lower than in the direct analysis. For example the detection limit of phenol acetate is 0.5 μg 1−1 and that of 4-nitrophenol is 10 μg 1−1. The acetylation method was also tested in the analysis of phenolic compounds from contaminated surface water samples.