As part of a study to assess potential regional surface-water acidification in the southern Blue Ridge region of the USA, more than 700 individual soils were sampled from 35 watersheds and analyzed. Sulfate adsorption isotherms were generated for all mineral soil horizons. Subsequent use of these data in dynamic watershed chemistry models required aggregation of adsorption data to two or three isotherms (one per soil mineral horizon) per watershed. The study evaluated several techniques for aggregation of the nonlinear adsorption-isotherm data. Results of the study show that aggregated-data values are extremely sensitive to the choice of aggregation procedure. The aggregated-coefficient and aggregated-point procedures appear to be straight-forward, reasonable approaches to data aggregation, but the aggregated functions generated by these procedures are usually biased; use of these procedures is strongly discouraged. In contrast, the aggregated-isotherm approach provided an effective means of aggregating data, even for very heterogeneous groups of isotherms; use of this procedure is recommended for aggregation of adsorption isotherms and for data for similar non-linear functions.