CCA-treated timber, in particular poles, appear to fail in service according to a well defined pattern of annular separations at the interface between latewood and earlywood in the timber year rings. The effect is marked, it is progressive, and the characteristic failure pattern Starts to appear in both ground and non-ground contact specimens after a few years of exposure. In this article, the phenomenon is dcscribed qualitatively, quantified and explanations for it advanced, through the findings of scanning electron microscope (SEM) and strength test investigations. The reason of such peculiarity, which is not observed in creosote or organic preservative treated timber, has initially a chemical reason and later a complex of anatomical, chemical and mechanical reasons.