Leaf-litter assays have advanced understanding of decomposition processes in both terrestrial and aquatic ecosystems. Some shortcomings inherent in the technique may be overcome through use of a cotton-strip assay. Key assumptions for using cotton strips as proxies for natural leaves are: 1) decomposition rates of the 2 materials are related, and 2) the materials decay in a similar way when exposed to the same environmental conditions. These assumptions were tested by comparing cotton-strip decomposition (loss of tensile strength and mass) and leaf decomposition (mass loss) across different floodplain habitats of the Tagliamento River (northeastern Italy). Patterns of loss of cotton-strip tensile strength and leaf mass were broadly comparable across river channels, ponds, and terrestrial sites. Differences between river channels and ponds were greater for loss of cotton-strip tensile strength than leaf mass, indicating that, in some situations, loss of cotton-strip tensile strength may be more sensitive to differing environmental conditions than loss of leaf mass. Loss of cotton-strip mass was less sensitive than loss of either tensile strength or leaf mass. Although combined data from all floodplain sites and additional sites in Swiss streams yielded a curvilinear relationship between loss of cotton-strip tensile strength and mass, the slope was extremely steep in the range of 20 to 30% mass loss (corresponding to 0 to 95% loss in tensile strength), indicating that inferring one variable from the other is unreliable. Leaf mass loss was significantly correlated with loss of tensile strength in fine- and coarse-mesh bags in ponds and in coarse-mesh bags in terrestrial sites. However, these correlations were relatively weak (r = 0.50–0.63), suggesting that loss of tensile strength did not accurately reflect leaf mass loss. Thus, the cotton-strip assay should not be used uncritically as a surrogate for leaf-litter assays, but it has potential as a standardized method to measure organic-matter decomposition in fluvial settings and as a functional indicator for stream assessment.