In this article, a compilation of resul ts on direct force measurements between colloidal particles in monovalent salts carried out with the colloidal probe technique based on Atomic Force Microscopy was presented. The interaction forces between similar and dissimilar particles was studied and it was concluded that, in general, these force profiles may be satisfacto- rily quantified by the DLVO theory down to distances of few nanometers. However, in the specific case where the charge of one of the involved particle is close to neutral, it was found that the sur- face potential of this particle may change its sign depending on the sign of charge of the opposite particle. In this respect, the assumption that the s urface potential of a particle is a property only related to the particle surface features and th e bul k prop ert ies is called int o que s tion.