This invention is related to a wood preservative composition comprising i) at least one active ingredient, and ii) a carrier system containing at least 10% w/w of a solvent selected from one or more ester compounds of the formula (I) wherein: R and R' are independently selected from Ci-Cβ-alkyl groups, and wherein the carbon chains of alkyl groups of 3 or more carbon atoms may be straight-chained or branched; and wherein the at least one active ingredient is substantially soluble in the earner system. The preservative compositions may be used to treat wood based materials, and the ester solvent may optionally be recovered following the treatment. Formula (I).