Adhesion between polystyrene particles in high purity water has been measured for a wide range of particle sizes. The results are compared with predictions based on the JKR model using literature values of the solid-vapor surface energy. When the surface roughness of the particles is accounted for, the JKR adhesion is still too large. An optimum value of the surface energy is calculated for one tip particle size and used for predicting the pull-off force for all of the substrate particle sizes, and it was found that this produces much closer agreement between theory and experiment although the scatter in the data is large. The mean pull-off force for all of the particle-particle measurements studied here is of the order of a few nanonewtons. Because of the large scatter in the pull-off force data, no clear trend in the pull-off force for different particle sizes has been observed. It is suggested that the effect of surface roughness on the pull-off force is significant.