The total organic carbon (TOC) content of aquifer materials has been found to have significant effects on the movement of pollutants in the subsurface environment. Accurate quantification of TOC is therefore of great importance to research in groundwater contamination. However, large discrepancies have been observed when laboratories employing different methods determine TOC on the same aquifer material. The study was undertaken to evaluate the extent of these differences and examine their causes. Five subsurface samples were distributed and analyzed for both total carbon and total organic carbon by several instrumental techniques at four laboratories. Results indicate that when the material is adequately prepared with regard to homogeneity and removal of carbonate minerals, reasonably comparable values are obtained for all the instruments used, with the exception of a wet oxidation technique that is insufficiently oxidizing. Trends in subsurface research indicate a need for methods with improved accuracy and precision to lower detection limits and increase reliability. (Copyright (c) 1989 American Chemical Society.)