The rheological behavior of solid/liquid suspensions is largely influenced by the size and concentration of the solid particles. This work deals exclusively with ceramic suspensions used in the Portuguese ceramic industry, i.e., several Portuguese clays and feldspars, and also suspensions made of mixtures of these two materials. The flow tests conducted showed that, generally, the suspensions had a pseudoplastic character, approaching Newtonian behavior as concentration decreased, although, in some cases, a real plastic behavior was also observed, as in the case of very fine clays with a wide particle size distribution and high solids content. Moreover, it was possible to relate, for each material, both the flow behavior index and the fluid consistency coefficient with the solids concentration and particulate size. As the particle diameter decreases the suspensions become less Newtonian: the flow behavior index decreases and the consistency coefficient increases. A similar trend was detected, for each material, with the increase of solids concentration. However, with the results obtained, it was not yet possible to establish an analytical correlation between those variables, but since the ceramic industry deals with very complex suspensions, additional information on the rheological behavior of the pastes is a step forward to more predictive flow calculations.