Using the particle interaction apparatus, adhesion forces between individual pairs of spherical polystyrene and gold particles (phi 2-20 mum) were measured. With the particle interaction apparatus the contact time and load can be adjusted keeping all other parameters constant. Adhesion forces between different pairs of particles varied by a factor of five even for similar particle sizes. Adhesion forces were significantly lower than expected from the JKR or DMT theory. AFM images confirmed that one reason was probably the surface roughness on the nanometer scale. For both materials the adhesion force did not depend on the load (at least up to loads of 1 muN). Thus plastic deformation is probably negligible. The adhesion force increased with contact time. We interpret this increase as being at least partially caused by squeezing out of surface films. By removing the surface film a closer contact between the particle surfaces is established.