Equipment and procedures were developed to study the conversion of lignocellulosic biomass to carboxylic acids using high-solids countercurrent fermentations. Countercurrent fermentations of cattle manure yielded a rapid fermentation (maximum 2.98 g of total acid/ [Ld]) with high acid concentrations (maximum of 32.5 g of total acid/L), but the acid yield tended to be low (maximum of 0.24 g of total acid/g of volatile solids). Countercurrent fermentations of a mixture of 80% municipal solid waste/20% sewage sludge fermented more slowly (maximum of 1.98 g of total acid/[L.d]) with a lower acid concentration (maximum of 26.5 g of total acids/L), but higher acid yields were achieved (maximum of 0.34 g of total acid/g of volatile solids).