Characteristic parameters of plastic deformation have been measured for Ni 3 (Al,Hf) single crystalline specimens deformed in compression over a wide range of temperatures (293-1100 K). These parameters are: (1) the resolved proof stress ([tau ] 0 . 2 % ) which is measured at 0.2% offset stress; (2) the corresponding work-hardening rate ([theta] 0 . 2 % ); (3) the microscopic activation volume (V 0 . 2 % ); and (4) the variation in the density of mobile dislocations ([Delta ][rho ] m /[rho ] m 0 . 2 % ) during transient tests such as load relaxations. The two last parameters were determined with a technique of repeated load relaxations. It is observed that not only [tau ] 0 . 2 % exhibits an anomalous behaviour with temperature but also [theta] 0 . 2 % and [Delta ][rho ] m /[rho ] m 0 . 2 % . A comparison of V 0 . 2 % ([tau ]) with similar measurements in Ni 3 Al shows that alloying effects alter the deformation mechanism.