The decomposition of 4-alkoxy-1,4-dihydro-2,3-benzodioxin-1-ols (1, Bd) in aqueous media was examined. Increasing the water content of the medium accelerated the decomposition of 1 and increased the formation of the corresponding 2-formyl benzoic acid ester (2) as the decomposition product. Electron spin resonance (ESR) studies using dimethylpyrroline N-oxide (DMPO) as a spin trapping reagent had revealed that hydroxyl radicals are formed during the decomposition of 1 (Matsugo et al., FEBS Lett., 184, 25 (1985)). Thus, water-mediated decomposition of 1 was suggested to occur, affording the ester 2 and hydroxyl radical. Direct involvement of water was confirmed by an 18O isotopic tracer experiment which revealed that 18O was incorporated exclusively into the formyl position of the ester 2. It is plausible that a hydrated hydroperoxide (5) is formed by the addition of water at the formyl position of the ring-opened structure of 1 at the initial stage of the decomposition of 1. Preliminary studies on the antibacterial activities of 1 showed moderate cell-killing activity, especially to Pseudomonas strains, and the activity was found to be related to the decomposition of 1.