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)] and zero-order dependence in [thiomorpholine] and [OH-]. 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 [OsO4(OH)2 ]2--thiomorpholine complex prior to the rate-determining regeneration of OsVIII by [Fe(CN)63-]. The values of the enthalpy and entropy of activation are calculated from rate constant values measured at different temperatures (20-40 ° C).