Borate autocausticizing can either replace (total autocausticizing) or supplement the kraft lime cycle (partial autocausticizing). Partial autocausticizing has been recently commercialized in one kraft mill and is undergoing trials in several additional mills. Although it is known that borate reacts with lignin, little information is available on the effect of borates on kraft pulp yield or fibre properties. The objectives of this study were to determine the effect of sodium metaborate, which is the borate product from autocausticizing, on kraft pulping and on the properties of black spruce pulps. The major findings of this study suggest that borate increases yield, can decrease rejects, improves the selectivity of lignin removal, does not affect the delignification rate and can increase pulp viscosity at the same kappa number.