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The concentration of chemical oxygen demand (COD), a common proxy for dissolved organic matter (DOM), was measured at seven drinking-water reservoirs and four streams between 1969 and 2006. Nine of them showed significant DOM increases (median COD change +0.08 mg L−1 yr−1). Several potential drivers of these trends were considered, including air temperature, rainfall, land-use and water sulfate concentration. Temperature and precipitation influenced inter-annual variations, but not long-term trends. The long-term DOM increase was significantly associated with declines of acidic deposition, especially sulfur deposition. Surface water sulfate concentrations decreased from a median of 62 mg L−1–27 mg L−1 since 1980. The magnitude of DOM increase was positively correlated with average DOM concentration (R 2 = 0.79, p < 0.001). Simultaneously, DOM concentration was positively correlated with the proportion of Histosols within the catchments (R 2 = 0.79, p < 0.001). A focus on the direct removal of DOM by water treatment procedures rather than catchment remediation is needed. [Copyright &y& Elsevier]