Some effects of second phase particles on grain boundary sliding have been observed in shear tests of bicrystal specimens. The majority of the test specimens were produced from an Al-0.05 % Fe alloy which yielded specimens containing second phase particles at the grain boundary only. Some specimens of pure Al and Al-4 % Cu were tested for comparison purposes. Sliding was observed to take place sharply at the grain boundary particles in the absence of grain boundary migration, but the ratio of sliding to crystal shear deformation was markedly reduced by the presence of grain boundary particles. Some evidence indicating a yield stress for the onset of sliding was obtained and the magnitude of this stress was related qualitatively to the density of grain boundary particle coverage. The mechanism through which the particles exerted their influence on sliding is also discussed.