By using Fenton chemistry (hydrogen peroxide, ferrous iron, pH<4) as an inter-stage treatment during production of Black spruce thermomechanical pulp, the specific energy consumption can be reduced by at least 20% to a constant tensile strength. The energy reduction can be achieved with preserved fibre length and cross-sectional dimensions. However, the fibre cross-sectional shape is affected, and fibres of the chemically treated pulps had the highest collapse index. A drawback with the acid hydrogen peroxide system is a decrease in pulp yield and a reduction in brightness. Different approaches for lowering the iron content of the chemically treated pulps prior to bleaching are presented. The discoloration is advantageously reduced (but not completely eliminated) by hydrogen peroxide bleaching.