Polydimethylsiloxane (PDMS) samples of different crosslink densities were oxidized in air plasma, and the stiffness change in the oxidized PDMS surface was monitored by performing tapping mode atomic force microscopy (TMAFM) distance-sweep measurements and numerical simulations of the cantilever equation of motion based on a contact mechanics model. The diffusion mechanism of hydrophobic recovery of an oxidized PDMS surface was examined by a combined use of TMAFM distance-sweep and phase imaging experiments. Our work shows that the modulus of the oxidized PDMS surface increases with increasing the oxidation time, and supports the diffusion mechanism of hydrophobic recovery. It is possible to extract information about the indentation force from observed indentation curves and to employ TMAFM for force modulation experiments at high modulation frequency. (C) 2001 Elsevier Science Ltd. All rights reserved.