Soft rot decay of treated wood is examined with special reference to hardwoods treated with CCA. Factors which adversely affect the chances of protection of hardwoods against soft rot are discussed. The ratio of the volume of the fibre cell wall to the volume of the fibre lumen is presented as a major factor influencing final preservative concentration in the fibre cell wall, the major strength contributing unit in wood. It is proposed that hardwoods are not necessarily more susceptible to soft rot than are conifers and that 0.2% Cu.wt/wt on gross wood will give control of soft rot in permeable hardwoods. Present and potential soft rot problems of treated conifers in North America are examined.