For several years, an extensive research program investigating new preservative systems has been ongoing at Mississippi State University. This paper details our continuing experience with field testing of copper naphthenate and other copper carboxylate preservative systems. Results from field stake tests at an AWPA Zone 4 test site are presented. In general, copper carboxylates made with 'synthetic' acids yielded results equivalent to or only slightly lower than systems with straight naphthenic acids or nap acids amended with synthetic neodecanoic acid bottoms. Oil carrier formulations outperformed those in water or light organic solvents. All systems carried in oil would qualify as ground contact preservatives at the proper retention. (c) 2004 Elsevier Ltd. All rights reserved.