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Development of high-value horticultural markets for compost requires improved characterization of compost properties. Most composts have pH of 7-8 and act as a partial substitute for lime in potting media. The objective of this research was to develop and validate a reproducible method for determining the pH buffering capacity of compost. To estimate compost buffering capacity (CBC), the quantity of acidity needed to reduce pH by one unit, dilute sulfuric acid (HSO; 0.1-1.0 mol H/kg compost) was allowed to equilibrate with compost at 22 °C for 72 h. The CBC was calculated as the negative reciprocal of the slope of the linear regression: compost pH (y-axis) versus acid addition rate (x-axis). The CBC for eight composts ranged from 0.21 to 0.45 mol H per kg compost per pH unit (avg = 0.31). The CBC determined from dilute acid addition for 72 h was validated via incubation of compost plus S° dust at 22 °C for 28 days. For the eight composts evaluated, S° addition to acidify compost by 1 pH unit (as predicted by CBC) resulted in an actual pH decline of 1.1 units (SE = 0.2), and predicted S° addition to acidify it by 2 pH units resulted in an actual decline of 1.7 units (SE = 0.2). The CBC test method is recommended for evaluation by commercial compost testing laboratories. [ABSTRACT FROM AUTHOR]