Tannins are believed to be particularly abundant in tropical tree foliage and are mainly associated with plant herbivore defense. Very little is known of the quantity, variation, and potential role of tannins in tropical leaf litter. Here we report on the interspecific variability of litter condensed tannin (CT) concentration among 16 co-occurring tropical rain forest tree species of French Guiana and explore the functional significance of variable litter CT concentration for litter decomposition. We compared some classical methods in the ecological literature to a method based on high-performance liquid chromatography (HPLC), coupled with CT degradation by phloroglucinolysis. The same litter was allowed to decompose in the field in the presence or absence of soil fauna. We found large interspecific differences in the average polymerization degree (2.7 to 21.3, for non-extractable CT) and concentration of litter CT (0-3.7% dry mass, for total CT) determined by HPLC, which did not correlate with Folin total phenolics but correlated reasonably well with acid butanol CT. The concentration and polymerization degree of HPLC-determined CT were the only variables of the multitude of measured initial litter quality parameters that explained a signi. cant amount of variation in litter mass loss among species, irrespective of animal presence. However, animal presence increased mean litter mass loss by a factor of 1.5, and the fauna effect on decomposition was best explained by a negative correlation with total HPLC CT and by a positive correlation with hemicellulose. Our results suggest that the commonly used acid butanol assay yields a reliable estimate of interspecific variation in CT concentration. However, the chemical structure of CTs, such as the polymerization degree, adds important information for the understanding of the functional role of CTs in litter decomposition. We conclude that the wide variation in structure and concentration of leaf litter CTs among tropical tree species is an important driver of decomposition in this nutrient-poor Amazonian rain forest.