an experimental optimization procedure has been used to investigate steam explosion of pinus radiata. enzymatic digestibility of this softwood is enhanced by steam explosion and addition of sulfur dioxide greatly improves effectiveness of the process. the effects of the variables time, temperature, and sulfur dioxide level have been described by empirical models. the response parameters modelled are: dry-matter yield; water-insoluble fibre yield; water-soluble sugar yield; enzymatic-hydrolysis sugar yield; and total sugar yield. near optimal total sugar yields were obtained with the conditions: 3 minutes; 215 $degree$ c; and 2. 55 sulfur dioxide. under these conditions the steam-exploded fiber was 82 digestible (cellulose to glucose) by the cellulase enzymes used, and a total sugar yield of 57 g/100 g o. d. wood was obtained. this study has shown that softwoods, of which pinus radiata is typical, can be made as amenable to hydrolysis by cellulase enzymes as hardwoods, which have previously been considered the only viable substrates for such processes. (author abstract) 24 refs