We report on the use of an optical fiber sensor for the determination of concentration of contaminants in solutions below the micromolar level based on the photothermal lens effect. A pump laser beam has been added to a conventional configuration widely used as refractometer, so that the device is improved to detect tiny changes of the refractive index of a sample related to the amount of heat generated following optical absorption. In this way, concentrations as low as 0.1 mu M of methylene blue in distilled water were measured. A simple experiment is also performed to demonstrate the photothermal effect. (C) 2014 Elsevier B.V. All rights reserved.