A technique for determining surface stress changes at solid-gas and solid-liquid interfaces with an atomic force microscope is presented. Therefore, the bending of a microfabricated cantilever, prepared with different opposite faces, is measured. This bending is directly proportional to a change of Delta sigma(1) - Delta sigma(2), where sigma(1) and sigma(2) are the surface stresses of the two faces. To demonstrate the possibilities and limitations of the technique (i) a reduction of the surface stress of silicon nitride upon the covalent binding of gaseous dimethyldichlorosilane was measured; (ii) an increase of the surface stress of silicon nitride in aqueous medium with increasing pH was detected; (iii) the unspecific adsorption of the protein bovine albumin in buffer was monitored. The accuracy of the method is better than 0.005 J m(-2). The main limit is residual drift of the cantilever deflection. As a consequence only changes of the surface stress which occurred more rapidly than approximate to 0.02 J m(-2) h(-1) could be reliably detected.