The substantial heterogeneity of dissolved organic matter (DOM) inhibits detailed chromatographic analysis with conventional detectors as little structural information can be obtained in the presence of extensive coelution. Here we examine the direct hyphenation of high-performance size exclusion chromatography (HPSEC) with nuclear magnetic resonance (NMR) spectroscopy to determine how size-distinguished fractions differ in composition. The results support the applicability of using HPSEC to generate more homogeneous fractions of DOM prior to NMR analysis and demonstrate that structure is significantly altered with size. The largest fractions are enriched in carbohydrate- and aromatic-type structures. The midsized material is substantial and is representative of carboxyl-rich alicyclic molecules (CRAMs). The smallest material has strong signatures of material derived from linear terpenoids (MDLT). Both CRAMs and MDLT have been recently hypothesized as major components of DOM, and detection by HPSEC-NMR confirms their existence as unique and separable entities. This preliminary work focuses on NMR hyphenation to HPSEC due to widespread use of HPSEC to characterize DOM. Online hyphenation is useful not only for time-efficient analysis of DOM but also for that of other highly complex samples such as those found in many environmental analyses.