A watershed acidification model for two New York state lakes was verified by simulating an additional year of field data that was excluded during the calibration period. The findings confirmed calibration and indicated that the most sensitive physicochemical and biochemical processes were chemical weathering, ion exchange, sulfate reduction by lake sediments, and sulfate adsorption by the terrestrial compartments. Long-term simulations showed that both lakes will reach steady state after a change in loading in fewer than 10 years. It was determined that 25 to 50% of the exchangeable bases in the upper soil will be depleted in the next 50 years without chemical weathering to resupply the base exchange complex. The sulfate sorption mechanism is the only one that could delay watershed response after a change in loading. Some alkalinity in one lake could be recovered if acid deposition was decreased by a factor of two. The other would remain alkaline even with twice as much acid deposition; however, in spring snowmelt events, lakes would become more acidic.