A method of analysis for monoesters of phthalic acid (‘monoesterphthalates’) in human urine has been developed. The method was needed to determine the hydrolysis and excretion efficiency of isotopically-labelled phthalate diesters (‘phthalates’) when they were fed to volunteers as part of a biomarker study to estimate total exposure to phthalates. The targeted substances were ¹³C-monobutylphthalate (MBP), ²H₄-monobutylphthalate (MBP), ²H₄-monobenzylphthalate (MBeP), ¹³C-monocyclohexylphthalate (MCHP), ¹³C-monoethylhexylphthalate (MEHP), and ¹³C-monoisodecylphthalate (MIDP). The monoesters in urine were deconjugated enzymatically, extracted into solvent, and then determined by high performance liquid chromatography - mass spectrometry (LC-MS) using atmospheric pressure chemical ionisation in the negative ion mode. The limits of determination were 10 ng ml⁻¹ for MBP, MCHP, MBeP and MEHP, and 40 ng ml⁻¹ for MIDP. The recovery from urine spiked at 100 ng ml⁻¹ was in the range from 70 to 85% except for MIDP which was lower at 55%. The between-batch reproducibility of the analysis was in the range 8 to 17% (n = 6 batches on separate days).