This study examined surface water samples for the presence of the monoaromatic hydrocarbons benzene, toluene, ethylbenzene and xylene. Water samples were collected from the sub-basin of the Tegas River and the Marrecas Stream microbasin located in the metropolitan region of the municipality of Caxias do Sul (Rio Grande do Sul State, Brazil). For all sampling points evaluated, only toluene was detected, with concentrations values in the range of 1.70-18.44 μg L(-1). In general, the relatively high toluene concentration for the samples collected were above the limit of 2 μg L(-1) established by the Brazilian government for Class I rivers for the protection of aquatic life. However, they were well below the established limit of 170 μg L(-1) in water used for human consumption.