E. coli NfsA is a highly promiscuous quinone and nitroaromatic oxidoreductase. Reactions follow a ping pong mechanism with the oxidative half-reaction rate-limiting. Reactivity of quinones or nitroaromatics correlates with one-electron reduction potential. Reactivity was generally lower for quinones than nitroaromatics having similar E17. Quinone reactivity pH dependence suggests a single-step hydride transfer mechanism.