In aqueous sodium hydroxide solutions at 25 °C, 3-penten-2-one undergoes retroaldol cleavage to acetone and acetaldehyde. The kinetics can be followed spectrophotometrically, and show two exponential phases with rather different rate constants. By a new analysis of the mathematical solution to this system we have been able to derive an improved method for calculating the microscopic rate constants. These rate constants arekhyd = 2.89 × 10−3 M−1 s−1,kdehyd = 4.04 × 10−4 M−1 s−1, andkretro = 3.36 × 10−4 M−1 s−1. The equilibrium constant for hydration of 3-penten-2-one (standard state for water is the pure liquid) is 7.13.