A new highly sensitive and environmentally friendly analytical method, using low-temperature partition extraction and ultra-high-performance liquid chromatography with tandem mass spectrometry, without the use of a labeled analyte, was developed and validated to determine and quantify urinary S-phenylmercapturic acid in urine samples. The World Health Organization, in its guidelines for air quality in Europe, recognizes that benzene is carcinogenic to humans and there is no safe level of exposure. Urinary S-phenylmercapturic acid is a sensitive and specific biological marker of exposure to benzene. The new analytical method, extraction, and analysis, were linear in the working range between 0.1 and 200.0 μg/L, precise (relative standard deviation lower than 6.0%), accurate (97.0-105.0%), and sensitive. The method's limits of detection and quantification were 0.02 and 0.084 μg/L, respectively. The recovery with the low-temperature partition extraction was 96.1%, with relative standard deviation less than 3.8%. The method is simple, accurate, and reproducible, and has been successfully applied in the evaluation of nonoccupational exposure to benzene, by urinary S-phenylmercapturic acid in urine samples.