In order to determine the mechanism whereby tannin renders copper-chrome-arsenic (CCA) preservative ineffective, blocks of * 2Pinus patula* 1 of low tannin content were (a) pretreated with 7.5% tannin solution (from <i>Acacia mearnsii</i>), dried at 95°C for 16 hours then treated with a 4% solution of CCA; (b) treated as for (a) but with a 2.5% tannin solution; or (c) treated as for (a) but with no tannin. Retentions of tannin solids and CCA were determined by mass/volume concentration methods and by atomic absorption. -- AATA