Abstract Acid rain research during the late 1970s and the early 1980s concluded that acid precipitation seriously affected the environment. It was, however, realised that humic substances (HS) in the water have an effect on the response of acid rain, and that HS acts as a modifier on both the chemical composition and on the biological activity. The HUMEX Project is studying the impact of HS on the acidification and the effect acidification has on the biological properties of HS. This is done by artificial acidification of a whole catchment. In the fall of 1988 a dystrophic lake was divided in two halves by a plastic curtain from the middle of the natural outlet to the opposite side. During the following two years, through September 1990, the water chemistry of the two lake halves was monitored. A number of scientists from Europe and North America have been studying the organic matter and the biota in the water and in the catchment area prior to the artificial acidification, which started in October 1990. The results, after 18 months of treatment, with a combination of sulphuric acid and ammonium nitrate, show a change in the water chemistry. In the experimental lake, there is a small increase in the concentration of S and organic N and a small decrease in color and pH. A reduction of the anion deficit in the treated basin is suggested to be due to a protonisation of the HS. There are also significant biological changes in the treated lake half.