Soluble methane monooxygenase (sMMO) from Methylocystis sp. M, consisting of hydroxylase, reductase and component B, catalyzes O-2- and NADH-dependent monooxygenation. Although rapid NADH-coupled turnover normally requires all three components (the sMMO system), three less complex systems (combinations of hydroxylase, reductase, NADH and O-2, or hydroxylase and H2O2 (H2O2/hydroxylase system)) were also functional. Using propylene as a substrate in the H2O2/hydroxylase system, addition of component B at all concentrations led to a decrease in the reaction rate. In the sMMO system, almost stoichiometric concentrations of component B were required to maximize the enzyme reaction rate; however, the reaction rate slowed when component B was present in concentrations greater than was required to saturate the initial hydroxylase. When straight C-chain alkanes longer than C4 (n-butane and n-pentane) were used as substrates, the distribution profiles of the products from the two systems differed. When iso-pentane was a substrate, product distribution differed between the two systems with or without component B, suggesting that component B might change the structure of the substrate-binding site of hydroxylase in existence of reductase. The degradation products and their proportions from a chlorinated alkene substrate (trichloroethylene) were identical when using the H2O2/hydroxylase and sMMO systems.