Ultraviolet absorption spectrometry was used to measure concentrations of bisulfite in thermal equilibrium with formaldehyde and hydroxymethanesulfonate in deaerated, argon- saturated aqueous solutions. The equilibrium constant of the title reaction was determined as a function of temperature in the range 288-318 K for pH values of 5-6, and at 298 K as a function of pH in the range 4.1-9.7. The temperature dependence (at pH 5-6) is ...