The invention relates to the sizing of paper and board using alkenyl succinic anhydride (ASA). The invention provides a sizing composition which comprises an aqueous emulsion of an ASA wherein the ASA incorporates a maximum of 1% by weight (preferably a maximum of 0.5% by weight) of polymeric residues. The composition may incorporate less than 0.5% by weight of olefins. Paper sizing compositions in accordance with the invention provide enhanced resistance to penetration of the edges of paper and board by agents such as hot hydrogen peroxide and lactic acid.