The transition from free-flowing to "stick-slip" or cohesive flow in particulate solids was studied using two systems: glass spheres in humidity-controlled air and iron spheres within a magnetic field. The flow characteristics of glass spheres in the range 25-3000 mum were studied as a function of relative humidity (RH) using a rotating inclined cup apparatus. It was found that there is a critical relative humidity at which the spheres undergo the transition from free-flowing to stick-slip behaviour, and this critical humidity increases with increasing particle size. Atomic Force Microscope (AFM) measurements suggest that this transition is due to the system achieving a certain critical level of cohesive interparticle force and not due to an abrupt rise in this force. Similar experiments were performed using iron spheres in a magnetic field, thus allowing the interparticle (magnetic) cohesive force to be controlled. The results confirmed the above finding and suggested that the transition from free-flowing to stick-slip behaviour occurs at a critical ratio of interparticle force to particle weight.