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Variation in dissolved organic carbon (DOC) concentrations of surface waters is a consequence of process changes in the surrounding terrestrial environment, both within annual cycles and over the longer term. Long-term records (1987–2006) of DOC concentrations at six catchments (0.44–10.0 km²) across a climatic transect in Scotland were investigated for intra-annual relationships to evaluate potential long-term seasonal patterns. The intra-annual mode of DOC export contrasted markedly between catchments and appeared dependent on their hydrological characteristics. Catchments in wetter Central Scotland with high rainfall–runoff ratios, short transit times and well-connected responsive soils show a distinct annual periodicity in DOC concentrations throughout the long-term datasets. Increased DOC concentrations occurred between June and November with correspondingly lower DOC …