Adhesional interaction forces between a single 5 mu m radius polystyrene sphere and an atomically smooth mica surface have been measured using an atomic force microscope. The pull-off force was determined as a function of two factors: the contact time between the surfaces at a constant maximum applied load and the applied lend. Careful analysis of the data in the contact zone revealed that the polystyrene underwent significant non-elastic deformations when the contact times were more than 10 s. These data were rationalized on the basis of viscoelastic deformation of the sphere as the load was applied: the lends used here exceeded the elastic limit for polystyrene. It appears from the data that the exact modelling of the sphere deformation is a complex problem involving both surface asperities and bulk material collapse. The pull-off force was seen to increase both with increasing applied load and with longer contact time. Calculation of the applied forces indicated that plastic deformation was likely, at least at the slower loading rates, and the relationship between the pull-off force and the applied load (P) agreed with a P-1/2 dependence as suggested by Maugis and Pollock. It was concluded that at the applied loads-used here, the surface asperities were effectively ＇squeezed out＇ resulting in an apparent smooth sphere contact.