Forest ecosystem response to acidification is being studied at the Bear Brook Watershed in Maine (BBWM) in a long-term, paired watershed experiment. The West Bear watershed has been treated with bimonthly additions of (NH4)2 SO4 since 1989. The adjacent East Bear watershed serves as a biogeochemical reference. Declining ambient SO4 deposition has driven the reduction of stream SO4 concentrations in East Bear from 106 μeq/L in 1989 to 85 μeq/L in 2003. Despite this reduction in ambient SO4 deposition, both watersheds have continued to acidify, and response has been greatest in the treated West Bear watershed. After four years of treatment, West Bear soil solution Ca concentrations reached a maximum of 140 μeq/L in 1993, over 2 times higher than East Bear concentrations. Soil solution Ca concentrations in West Bear declined after 1993 and reached pre-treatment levels by 2001. In contrast, soil solution concentrations of NO3, Al, SO4 and H in West Bear have remained elevated compared to East Bear. Changes in soil solute concentrations were paralleled by patterns of solute concentrations in streamwater, reflecting a shift from base cation to Al buffering mechanisms. Periodic sampling of tension lysimeters has been used to track these changes in soil solutions throughout the 19-year experiment and recent event-based sampling adds to our understanding of the biogeochemical mechanisms of acidification at the BBWM.