The potential of high temperature steam, dilute acid and different reaction times to optimize the nutritional value of lignocellulosic by-products, for both ruminant and monogastric feed, was investigated. Wheat straw was the lignocellulosic model used. Neutral and acid detergent fibre compositional data (and hence hemicellulose by difference) from wheat straw treated by a matrix of 36 diluteacid treatments (four H2SO4 concentrations, 0, 6, 12 and 18 g kg-1 DM straw; three temperatures, 98, 121 and 134-degrees-C; three reaction times, 40, 80 and 120 min) were used to compare the effects of the different physico-chemical treatments. Improvements in potential nutritional utilization for ruminants was estimated by in vitro gas production and in sacco (sheep) degradation; for monogastric animals, carbohydrate solubilization of treated straw samples by polysaccharidase preparations was used to gauge the effectiveness of the treatments. The predominant effect of dilute-acid treatments was to solubilize the hemicellulose fraction of wheat straw, the harshest treatment used (18 g sulphuric acid kg-1 DM, 120 min, 134-degrees-C) achieving 67\% hemicellulose solubilization. All three variables (acid concentration, temperature and reaction time) affected extent of hemicellulose solubilization with acid concentration and temperature being the most important. Hemicellulose solubilization was highly correlated to each of the biological parameters of potential feed value. Relating decrease of hemicellulose content of treated straw (18 g sulphuric acid kg-1 DM, 120 min, at 98, 121 and 134-degrees-C) with rate of gas production, in sacco degradability after 96 h and enzymic carbohydrate solubilization after 48 h gave correlation coefficients of -0.94, -0.99 and -0.98, respectively. Treated straw was used undried or after drying at 45-degrees-C or 105-degrees-C in the biological evaluation assays; both in vitro and in sacco measurements were affected by drying conditions. Using straw that was undried, dried at 45 and 105-degrees-C gave progressively decreasing utilization by rumen microbes (P < 0.05).