We report a second catalytic activity of Pseudomonas aeruginosa arylsulfatase (PAS). Besides hydrolyzing sulfate monoesters, this enzyme catalyzes the hydrolysis of phosphate monoesters with multiple turnovers (> 90), a k(cat) value of 0.023 s(-1), a K-m value of 29 mu M, and a k(cat)/K-m ratio of 790 M-1 s(-1) at pH 8.0. This corresponds to a remarkably high rate acceleration of 1013 relative to the nonenzymatic hydrolysis [(k(cat)/K-m)/k(w)] and a transition-state binding constant (K-tx) of 3.4 pM. Promiscuous phosphatase and original sulfatase activities only differ by a factor of 620 (measured by k(cat)), so the enzyme provides high accelerations for both reactions. The magnitudes and relative similarity of the kinetic parameters suggest that a functional switch from sulfatase to phosphatase activities is feasible, either by gene duplication or by direct evolution via an intermediate enzyme with dual specificity.