Wood ash application has been suggested as a means to mitigate acidification of surface waters. However little is known about the effects of wood ash to freshwater organisms. A Clark-type oxygen electrode was used to investigate the effects of wood ash, cadmium and aluminum on the respiration of the freshwater amphipod Gammarus pulex (L.). Harmful effects following wood ash addition were primarily found to be explained by elevated pH. At high concentrations (> 1g/l) of wood ash the respiration of G. pulex decreased when no adjustment of pH was made, a response that was negatively correlated to the elevated pH (r = −0.43). An ion-related response was also detected in the pH-adjusted (pH 7) tests, although it was not statistically significant. Cadmium proved to be sublethal in the range of 0.2–0.4 mg/l in a 24-h exposure. Mortality was 100 % at 0.5 and 1.0 mg Cd/l. No effects were detected after exposure to Al, due to the high pH (pH 7–8) of the solutions and the subsequently low concentrations of labile monomeric Al. We demonstrate that the method described in this paper could be a useful probe for the detection of sublethal concentrations of contaminants in freshwater ecosystems.