Abstract: An improved method for purifying dark colored chlorine containing effluents from a chlorine based pulp mill bleaching plant is disclosed, wherein the purification process involves treating the effluent in a bed containing a particulate weakly basic anion exchange and/or absorbent resin which takes up the colored lignin degradation products in the effluent until the resin has been saturated with pollutants to a given degree, whereupon the resin bed is eluted by means of an alkali solution. At least a fraction of the eluate which is released from the resin bed during the elution is acidified to a pH of less than 2, so that the majority of the total solids of the acidified effluent is precipitated, the precipitate is separated from the residual acidic liquor which contains the majority of the inorganic chlorine of the acidified eluate, and the precipitate then may be suitably transferred to a recovery plant of the pulp mill for eventual disposal.