A critical load is the amount of atmospheric pollutant that can be deposited on a sensitive ecosystem without causing measurable, long-term degradation in ecosystem form or function. The authors compare several methods for making critical load estimates of SO4 deposition to lakes. The models were originally developed to address surface water acidification issues. Critical loads are computed here as the levels of deposition required to reduce surface water acid neutralizing capacities (ANC) from present values to either 25 microeq/L or 0 microeq/L. These end points were selected to provide a common basis for comparing critical load values obtained from the various models and are not intended to provide definitive critical load estimates. Differences in results are observed not only between models, but within models when slightly different approaches are used for estimating certain parameters. The interpretation of model results depends strongly on how systems are grouped regionally.