The aim of this study was to quantify the toxic compounds present in steam-treated wheat straw and measure their effects on rumen microbial activity. Wheat straw was impregnated with 1.8% H2SO4 and subjected to steam treatment at either low temperature (134-degrees-C) for 100, 200 and 300 min or high temperature (210-degrees-C) for 1.5, 3, 4.5 and 6 min. Samples were analyzed for carbohydrate composition, total and extractable phenolics, tannin, furfural and hydroxy-methylfurfural (HMF). Utilization of steam-treated samples by rumen microbes were assessed by gas production. At both treatment temperatures significant hemicellulose hydrolysis (80.1 and 96.7%), formation of furfural (0.31 and 1.24%) and deploymerization of lignin, measured as extractable phenolics (5.1 and 9.0%), occurred. There was no evidence that furfural was toxic to rumen microbes. However, the presence of extractable phenolics lowered total gas production.