Immobilised DNA-oligo layers are scientifically and technologically appealing for a wide range of sensor applications such as DNA chips. Using microcantilever-based sensors with integrated readout, we demonstrate in situ quantitative studies of surface-stress formation during self-assembly of a 25-mer thiol-modified DNA-oligo layer. The self-assembly induces a surface-stress change, which closely follows Langmuir adsorption model. The adsorption results in compressive surface-stress formation, which might be due to intermolecular repulsive forces in the oligo layer. The rate constant of the adsorption depends on the concentration of the oligo solution. Based on the calculated rate constants a surface free energy of the thiol-modified DNA-oligo adsorption on gold is found to be -32.4 kJ mol(-1). The adsorption experiments also indicate that first a single layer of DNA-oligos is assembled on the gold surface after which a significant unspecific adsorption takes place on top of the first DNA-oligo layer. The cantilever-based sensor principle has a wide range of applications in real-time local monitoring of chemical and biological interactions as well as in the detection of specific DNA sequences, proteins and particles. (C) 2002 Elsevier Science B.V. All rights reserved.