The partial enrichment of a new enzyme, dimethylmaleate hydratase from Clostridium barkeri and some of its characteristics are described. The unstable and oxygen-sensitive hydratase depends on ferrous ions and is induced during growth of C. barkeri on nicotinic acid. The enzyme uses both dimethylmaleate and the hydration product, 2,3-dimethylmalate, as substrates to establish an equilibrium that is 70% in favour of the latter acid; dimethylfumarate is not attacked. A 2,3-dimethyl[3-3H]malate specimen was prepared from dimethylmaleate with the hydratase in tritiated water. Based on proton attack at the re-face of the double bond, experimental results indicate the (2R,3S)-configuration for this malate. The hydration reaction takes an anti-course. The tritium label was lost in the sequence (2R,3S)-2,3-dimethyl[3-3H]malate----(R)-[2-3H1]-propionate----(2R) - [2-3H1]propionyl-CoA----(2S)-methylmalonyl-CoA. This result confirms the stereochemical course of the 2,3-dimethylmalate lyase reaction, inversion of configuration, by an independent approach. The hydratase reaction completes the degradation scheme of nicotinic acid by C. barkeri. The pathway is briefly reviewed.