The present invention provides a wood preservative formulation comprising an isothiazolone, an organic fungicidal timber decay preservative and an unsaturated carboxylic or sulphonic acid, salt or precursor thereof. The formulations of the invention are surprisingly effective at protecting wood and other cellulosic substrates, in particular at providing prolonged protection against in-service surface staining. The invention also provides methods for treating wood and other cellulosic substrates with said formulations.
