Rate and equilibrium constants have been measured for the hydration and retroaldol reactions of cinnamaldehyde. The equilibrium constant for the 1,4-addition of water to cinnamaldehyde is 4.42 × 10−3. The rate constants for hydroxide catalyzed reaction, extrapolated to zero hydroxide concentration (to correct for the addition of hydroxide to the aldol carbonyl), are: [Formula: see text];[Formula: see text]; and [Formula: see text]. The rate of the formation reaction was measured by adding small amounts of acetaldehyde to alkaline solutions of benzaldehyde: [Formula: see text] and Koverall = 1480 M−1. The course of the synthetically useful reaction of acetaldehyde with benzaldehyde is discussed in the light of these results.