Ultraviolet combustion of lake waters combined with sensitive spectrophotometric NO3 + NO2‐N and NH4‐N analyses offers a 100‐fold increase in sensitivity over the micro‐Kjeldahl method for total dissolved organic nitrogen. The procedure permits quantification of two organic nitrogen fractions in lake waters based on the relative rate and amounts of labile NH4‐N and refractory NO3 + NO2‐N released. Combustions of water from six lakes under uniform conditions disclosed widely different amounts of the two fractions and increased amounts of the refractory fraction with increased depth in one lake.Duration of combustion, pH, and adequate oxygen affected ultraviolet combustions of freshwater samples and must be controlled to achieve uniform results. A recommended procedure for freshwaters consists of 1.5–3 hr of combustion with continuous oxygenation at pH 4.0 buffered with boric acid.Additions of organic carbon as glucose and glycine or tryptophane to standard solutions and natural lake samples demonstrated that the rate of nitrogen release and the final combustion product expression were a function of: 1) the molecular nature of the dissolved organic nitrogen compound (DONC), 2) the concentration of total dissolved organic carbon (TDOC), and 3) the inorganic salt concentration.Use of this procedure in comparing lakes of different trophic status should permit investigation of the role of DONC and its decomposition in primary production processes of lake ecosystems.