In humid regions, the chemical flux and cycling of elements is intimately linked to the hydrologic cycle. This insight opened in the late s. XX a worldwide avenue for the use of small watersheds as ecological units to study the hydrological and biogeochemical functioning of ecosystems at the small catchment scale. The Montseny catchment research, starting in 1978, initially addressed the forest response to acid rain. But continuous recording for about 4 decades in two small catchments allowed to describe the changes in streamwater chemistry related to changes in atmospheric deposition (with particular emphasis to S, N and P deposition), to climate change and to the inputs of African dust. Further research and new hypothesis testing may take advantage of the collected data series in these long-term study sites at a Mediterranean site. This is the motivation for the publication of the quality-checked original stream and atmospheric deposition chemistry files whose links accompany this paper.