Batch culture experiments examined the ability of an isolated bacterial community to utilize four humic substances with similar molecular size but variable elemental composition. Univariate and multivariate linear regression analysis of the results provided evidence of a significant positive relationship between the N: C ratio and bacterial concentrations. In contrast, neither H: C nor O: C ratios were significant predictors of humic substances bioavailability, and their inclusion in the multivariate model provided no further explanatory power compared with the univariate model using N: C as the single independent variable. These findings suggest that N: C ratios provide the best indicator of bioavailability for complex, recalcitrant carbon moieties typical of many aquatic systems.