Quaternary ammonium carbonates having the formula ##STR1## wherein R.sup.1 is a C.sub.1 -C.sub.20 alkyl or aryl-substituted alkyl group and R.sup.2 is a C.sub.8 -C.sub.20 alkyl group have been synthesized.These compounds and compositions further comprising the corresponding quaternary ammonium bicarbonate ##STR2## wherein R.sup.1 is the same or a different C.sub.1 -C.sub.20 alkyl or aryl-substituted alkyl group and R.sup.2 is the same or a different C.sub.8 -C.sub.20 alkyl group and/or the corresponding quaternary ammonium metal carbonate ##STR3## wherein R.sup.1 is the same or a different C.sub.1 -C.sub.20 alkyl or aryl-substituted alkyl group; R.sup.2 is the same or a different C.sub.8 -C.sub.20 alkyl group and M is a non-coupler metal, are prepared by reacting two reactants, a C.sub.1 -C.sub.20 alkyl or aryl-substituted alkyl, C.sub.8 -C.sub.20 alkyl quaternary ammonium chloride and a metal hydroxide, in a solvent comprising a C.sub.1 -C.sub.4 normal alcohol. The amount of metal chloride reactant is that amount sufficient to yield the C.sub.1 -C.sub.20 alkyl or aryl-substituted alkyl, C.sub.8 -C.sub.20 alkyl quaternary ammonium hydroxide, a metal chloride, and optionally unreacted metal hydroxide. The resultant quaternary ammonium hydroxide and any unreacted metal hydroxide are then reacted with carbon dioxide to yield the quaternary ammonium carbonate and optionally metal carbonate.Also provided is a method for preserving a wood substrate. Accordingly, the substrate is treated with a metal coupler-free wood preservative system comprising a biocidal effective amount of the carbonate quats above, preferably those prepared by the method above, and a solvent.