Several commercial thermomechanical pulps and stone ground-wood pulps were bleached to determine how the bleaching rate is affected by peroxide and alkali concentration, pulp consistency and temperature. The bleaching rate seems to be determined by the balance between the bleaching action of peroxide anion (HO) and color formation by alkali (OH). At a given peroxide concentration, increased alkalinity increased the bleaching rate due to the formation of peroxide anion, up to a given alkali concentration above which a further increase led to a decrease in reaction rate. This is probably due to the fact that at high alkalinity, pOH < 2.5, an increase in hydroxide ion concentration gives only a slight increase in the peroxide anion concentration, at lower alkalinity, the increase in peroxide anion concentration is approximately proportional to that of the hydroxide ion.