In order to optimize the chemical pretreatment of a high yield sulphite pulping operation, thirty-six pilot scale pulping runs have been conducted according to a central composite design. Second order polynomial models for digester yield, chip sulphonation as well as for spent liquor oxygen demand and toxicity have been obtained with adjusted R2 fits above 75%. Optimization results indicate that high chip sulphonation can be attained most efficiently at low initial pH with low SO2 concentrations. Increases in spent liquor toxicity associated with SO2 concentrations in excess of 3.5% can be offset by performing chip sulphonation at high pH.