Two methods are described for determination of organic carbon, inorganic carbon, and total nitrogen in sediments, sediment trap materials, and plankton. Both methods discriminate organic and inorganic carbon by acidification, avoid losses of acid‐soluble organic and inorganic matter, and utilize an automated CHN analyzer for all elemental determinations. Short‐term precisions for organic carbon and total nitrogen are about ± 1% of the measured value with blank levels < 1.5 µg for each element.