Forest harvest residues can be a cost-effective feedstock for a biorefinery, but the high lignin content of forest residues is a major barrier for enzymatic sugar production. Sulfite pretreatment to overcome strong recalcitrance of lignocelluloses (SPORL) was applied to a Douglas-fir (Pseudotsuga menziesii (Mirb) Franco var. menziesii) forest residue in a range of sulfite and acid loadings at 165 degrees C for 75 mm with liquid to wood ratio of 3:1. Sodium bisulfite and sulfuric acid charge as mass fraction of oven dry biomass of 12% and 2.21%, respectively, was optimal in terms of enzymatic cellulose saccharification, sugar yield and formation of hydroxymethylfurfural (HMF) and furfural. Enzymatic glucose yield was 345 g kg(-1), or equivalent to 82.3% of theoretical at a cellulase (CTec2) dosage of 15 filter paper unit (FPU) per gram of glucan. HMF and furfural formation were low at approximately 2.5 g L-1 each in the pretreatment hydrolyzate. Delignification was important to achieve good cellulose saccharification efficiency, however, approximately 80-90% hemicellulose removal is also required. Substrate enzymatic digestibility (SED) was found to correlate to a combined parameter Z(CHF) of delignification and hemicellulose dissolution well, suggesting that the combined hydrolysis factor (CHF) - a pretreatment severity measure - can be used to predict saccharification of forest residue for scale-up studies to reduce numbers of experiments. Published by Elsevier Ltd.