Dissolved organic carbon (DOC) in water samples can be freeze dried and combusted to CO2 at 580°C in sealed, evacuated tubes for carbon isotopic analysis. Dissolved platinum catalyst is added to promote combustion, and CuO is used as an oxidant. Freshwater DOC samples from Massachusetts ponds and rivers have d13C values of -26 to -31%o, and values for surface seawater from Woods Hole Harbor and the Gulf of Maine are -21 to -24%o.