Abstract In this work were determined triclosan and 4n‐nonylphenol in water from a reservoir that is used to provide water to a purification plant in an important city in C olombia. The analytical methodology was validated using solid‐phase extraction and analysis by gas chromatography‐mass spectrometry ( GC‐MS ). The analysis by GC‐MS showed good linearity in the range of 0.05–5 μg/ L for both compounds. Recoveries from 79 to 109% and standard deviations of 2.5–7.7 for low concentrations and from 3.8 to 9.6 ( n = 5) for high concentrations were obtained for both compounds. In C olombia, this is the first time that these compounds have been analysed in water supplying of a drinking water treatment plant. The validated method was applied to the analysis of 27 samples collected in August 2010 in 11 locations from the reservoir and in the influent and effluent of the drinking water treatment plant. In total, seven samples were found to contain triclosan.