Thiomorpholine (tetrahydro-1,4-thiazine), in the presence of Os(VIII) as catalyst, is oxidized by alkaline hexacyanoferrate(III) to 2-hydroxythiomorpholine. There is a first-order dependence in [Fe(CN)63 and [Os(VIII)J and zero-order dependence in [thiomorpholine) and fOHj. The observed rate constant was dependent on [Fe(CN)64] the type of electrolyte and the permitivitty of the reaction medium (10-40% ethanol). The proposed mechanism includes the formation of a transient [0s04(OH)2 ]2-thiomorpholine complex prior to the rate—determining regeneration of Os” by [Fe(CN)63i. The values of the enthalpy and entropy of activation are calculated from rate constant values measured at different temperatures (20-40 °C).