The viscosities of unbleached kraft, kraft-AQ, soda, and soda-AQ pulps were determined over a wide range of additive concentrations and sulphidities, but at constant effective alkali and H-factor. There was no significant effect of AQ or sulphide ion on viscosity. As expected, effective alkali had a large effect on viscosity, while H-factor had a lesser influence. Viscosity is not a reliable indicator of the strengths of unbleached pulps made by reductive alkaline processes.