An agricultural residue, sugarcane bagasse (Saccharum spp., hybrid), was used in this study and was stored in piles for 3-26 weeks. Analytical pyrolysis of fresh bagasse samples and samples taken from the center of the bagasse piles after 3.25, 6.5, 13 and 26 weeks of storage showed only small changes in their chemical compositions (pentosans, hexosans and lignin). However, samples taken from the outer surface layer (pile crust) had significantly lower contents of pentosans and hexosans and higher contents of lignin compared to the fresh samples. Samples taken from microbially degraded portions of the pile (composting fraction) showed preferential degradation of the pentosans relative to the hexosans and lignin. The pentosan content of the composting fraction was significantly lower than that from the fresh samples and samples from the centers of the piles. Because of the preferential degradation of the pentosans, the hexosans and lignin contents were relatively higher for the composting fraction compared to the fresh samples. These results clearly indicate that most of the deterioration observed in the bagasse piles occurred on the outer surface layers. Catalytic upgrading of the bagasse pyrolysis vapors also showed that the yield of hydrocarbons from the pile crust was 23% lower than that for the fresh material. However, there were no significant differences between the yields of hydrocarbons from the fresh samples and samples taken from the center of the pile after 3.25, 6.5, 13 and 26 weeks of storage.