Athermal solid solution strengthening was examined in substitutional Nb(Cb)-Hf, Nb-W, and Nb-Ta alloys deformed at 900 K. Ultrahigh vacuum degassed single crystals were tested in compression. The concentration dependence of the yield stress is nearly linear in Nb-Hf and Nb-W alloys for the range of compositions tested (< 6 at. pct). The rate of hardening with solute addition was the greatest for the Nb-Hf alloys, considerably less for the Nb-W alloys, and the least for the Nb-Ta alloys which exhibited little strengthening. The results indicate that athermal strengthening in these alloys appears to be primarily dependent on the atomic size misfit between solvent and solute atoms and not on the elastic modulus mis-fit. Results on the slip behavior are also presented.