Under mild alkaline condition the bleaching activator tetraacetyl ethylenediamine (TAED) forms peroxyacetic acid with sodium perborate and improves the brightness of thermomechanical pulp. The activator makes it possible to bleach the pulp efficiently even without addition of sodium hydroxide at any temperature in the range 40 to 70 degrees C. A mathematical model has been proposed to allow estimation of the total peroxy acid consumption during activated bleaching process. Using this model it is possible to calculate the extent of peroxy acid decomposition by predetermining the rate constant and activation energy. The effect of bleach activation was found to be maximum at temperatures below 60 degrees C. The influences of bleaching time, pH, consistency, temperature and TAED charge on the bleach activation has been evaluated. The activator is effective enough to improve the brightness of sodium perborate bleached pulp at an average charge of about 50 mass% of sodium perborate. Apparently, high consistency dispersive bleaching without free alkali is a technological and economic advantage of the process.