The objective of these experiments was to develop a simple in vitro technique for evaluating the production and neutralization of acid as feeds ferment in the rumen. An in vitro approach was adopted to eliminate animal factors. The procedure was based on the method of Tilley and Terry, with some modifications developed in this project. Residual acidity (acidogenicity value) was determined by the dissolution of Ca from CaCO3 powder added to the media at the end of 24-h incubations. Acidogenicity values (AV) were higher when 20% strength buffer was used, while lowering buffer pH increased values, equally across all feeds. There was no effect of donor animal diet, but considerable day-to-day variation in the fermentation activity of rumen fluid. This variation likely reflected the substrate preferences of differing microbial populations, so that several standard feeds may be required to account for this effect. A series of 28 diverse feed ingredients was evaluated for AV using a mixture design, with 85 combinations of ingredients: 100% of each ingredient (n = 28); 50% of each ingredient and an equal mixture of all others (n = 28); equal mixture of all ingredients, excluding one (n = 28); and an equal mixture of all ingredients (n = 1). The effects of most ingredients on AV were essentially linear, though some extreme ingredients showed nonlinear effects. Protein sources had low AV, forages intermediate AV and starchy feeds high AV. Calcium contained within feeds contributed to AV, particularly for legumes, sugar beet pulp, and citrus pulp, and must be accounted for.