A technique is proposed for biomonitoring of sporadic acidification events in rivers. Individuals of the bioindicator species are pre-incubated with a chemical marker, then transplanted to the test location for the period of interest, then analysed for marker content; acidification events are inferred on the basis of the degree of reduction in marker concentration. To assess the validity of the proposed technique, we performed laboratory trials with the aquatic bryophyte Fontinalis antipyretica Hedw. as bioindicator species, and cadmium (Cd) as marker. The bryophytes were pre-incubated with a Cd solution, obtaining saturation concentrations in the extracellular compartment and near-saturation concentrations in the intracellular compartment. In a first series of experiments, Cd-preloaded bryophyte apices were maintained for 1-30 h in water of different pH; the results clearly indicated that the amount of Cd released is dependent on pH. In a second series of experiments, Cd-preloaded bryophyte apices were maintained for 1-24 h in water with various combinations of pH and aluminium (Al) concentration (Al being a highly toxic element that is typically mobilized in acid waters). In these experiments, pH again had a marked effect on the amount of Cd released, while Al concentration had no consistent effects. To facilitate data analysis, Davidson-type equations were fitted to the results of the second series of experiments, allowing prediction of medium acidity on the basis of Cd concentration remaining in the extracellular and intracellular compartments.