Abstract: The invention relates to acidification of soap, wherein the acidification is performed with a sodium bisulphite solution produced by absorbing the SO.sub.2 entrained in the flue gases obtained from the combustion of odor gases derived from a chemical pulp mill into an alkaline sodium solution which is NaOH or oxidized white liquor, or with a sodium bisulphite solution which is NaHSO.sub.3 -containing mother liquid obtained from SO.sub.2 or NaHSO.sub.3 acidification of soap. The acidification can be conducted in one or more steps and by the cross-current and/or counter-current method by recycling the Na.sub.2 SO.sub.3 mother liquid obtained from the acidification.