Two headwater streams with low DOC and different pHs (4·5–4·8 and 5–6·5) were acidified with H 2SO 4 to pH 4·1 and 4·5, respectively, for 24-h periods. Neutralization of the added acid occurred by protonation of ANC (HCO 3-dominated in the higher pH stream), desorption of Ca (<15 μeq litre −1) and Mg (<6 μeq litre −1), and desorption and dissolution of AL (<250 μg litre −1) from the stream bed. The concentrations of dissolved organic carbon (DOC) remained constant within the experimental reaches. The concentrations of Na, K an H 4SiO 4 also remained constant, indicating no detectable increase in the rate of chemical weathering in the stream bed. After acid addition was stopped, concentrations of Ca, Mg and Al decreased to below background, indicating reversible ion exchange as the principal mechanism for the mobility of Ca and Mg and to a lesser extent for Al. Repeated acidifications indicated that significant regeneration of cations on the exchange surfaces of the stream substrate occurs rapidly.