Two types of mesophilic methanogenic granules (R-and F-granules) were developed on different synthetic feeds containing acetate, propionate and butyrate as major carbon sources and their metabolic properties were characterized. The metabolic activities of granules on acetate, formate and H2-CO2 were related to the feed composition used for their development. These granules p e r f o r m e d a reversible reaction between H2 production from formate and formate synthesis from H2 plus bicarbonate. Both types of granules exhibited high activity on normal and branched volatile fatty acids with three to five carbons and low activity on ethanol and glucose. The granules performed a reversible isomerization between isobutyrate and butyrate during butyrate or isobutyrate degradation. Valerate and 2-methylbutyrate w e r e produced and consumed during propionate-butyrate degradation. The respective apparent Km (mM) for various substrates in disrupted R-and F-granules was: acetate, 0.43 and 0.41; propionate, 0.056 and 0.038; butyrate, 0.15 and 0.19; isobutyrate, 0.12 and 0.19; valerate, 0.15 and 0.098. Both granules had an o p t i m u m temperature range from 40 to 50°C for H2-CO2 and formate utilization and 40 ° C for acetate, propionate and butyrate utilization and a similar o p t i m u m pH.