We analyze the stressing of a crack along a grain interface on which there have segregated solute atoms which retain a residual misfit volume relative to atoms of the host solid. Results suggest that the critical stress intensity factor necessary to emit a dislocation from the crack tip is increased in proportion to the misfit volume, so that oversized solutes tend to pin crack tip dislocations. This is a complementary embrittling mechanism to the often discussed effects of segregants on reducing the reversible work of interfacial separation. It is consistent with empirical correlations of the embrittling potency of solutes with their atomic size differences relative to the host, e.g., as for Sb, Sn, and S in Fe.