The temperature, strain-rate and orientation dependences of strength are investigated in single crystalline Ni₃(Al, 5.0Si), and the results are compared with those on Ni₃(Al, 2.5Si) single crystals reported previously by the present authors. The magnitude of the anomalous positive temperature dependence of strength is found to increase with increasing Si additions and the value of activation energy to yield the mechanical anomaly is found to be in good accordance with that being predicted from the data for polycrystals. For an orientation near [\bar111], where the cube slip governs the plastic behaviour above the peak temperature, the temperature and strain-rate dependences of strength are found to differ considerably from what was found in Ni₃(Al, 2.5Si). This is shown to be caused by the difference in the deviation from stoichiometry and not by the difference in Si content of the alloys. On the other hand, the steady-state deformation behaviour being governed by the octahedral viscous flow for the orientation near [001] resembles that in Ni₃(Al, 2.5Si), where the stress exponent as well as the activation energy for the deformation is found to be nearly identical. The activation energy evaluated is of the same order as that for diffusion of Si in Ni₃Al.