Abstract: Fungicides for preservation of wood, such as tributyltin oxide (TBTO) and tributyltin naphthenate (TBTN), are stabilized through inhibition of a normal autooxidation by addition of up to 1% of at least one metal salt of formula (I), wherein R is C.sub.4 -C.sub.16 alkyl, aryl or alkylaryl, optionally substituted with oxygen, nitrogen, sulfur or halogen. R.sub.1 is C.sub.1 -C.sub.4 alkyl, n is 0 or 1, and Me is calcium, magnesium, nickel, zinc, copper or cadmium when n is 0, and tin when n is 1. This results in significant inhibition of the degradation of the fungicide which takes place after application to the wood so that the fungicide maintains its activity for an extended period of time.