Polyester matrix composites reinforced using nonwoven coir or oil palm empty fruit bunch fiber mats were manufactured. Fibers were used unmodified, chemically modified by acetylation, or treated with silane or titanate coupling agents. Composite test pieces were exposed to decay fungi in unsterile soil for up to 12 months, along with samples made of unreinforced, or glass fiber reinforced, resin. Water exposure tests were also performed. The effect of such exposure on the mass loss, tensile and flexural properties of the samples was evaluated. Mechanical properties deteriorated as a result of exposure. However, acetylation of fibers, or treatment with silane coupling agent was found to afford a significant degree of protection. © 2000 John Wiley & Sons, Inc. J Appl Polym Sci 77: 1322–1330, 2000