Two RuIV=O catalysts with either a pentadentate bispidine ligand L1 or a bidentate pyrazolate L2/terpy L3 combination of ligands have very different efficiencies as oxygen transfer catalysts for the selective oxidation of sulfides to sulfoxides: the [RuII(L1)(solvent)]2+/iodosyl benzene system has an initial TOF of approx. 40 h-1 and quantitative yield, with [RuII(L2)(L3)(solvent)]+ the initial TOF is approx. 12 h-1 with a maximum yield of approx. 60%. By experiment (cyclovoltametry) it is shown that there is S- to O-linkage isomerization of the RuII sulfoxide product complex, and this may partially switch off the catalytic cycle for the L2/L3-based catalyst. It emerges that the reasons for the reduced efficiency in the case of the L2/L3, in comparison with the L1-based catalyst, are a more efficient linkage isomerization, a more stable S-bonded, in comparison with the O-bonded, RuII-based isomer, and inefficient ligand exchange in the product (hydrolysis produces the free sulfoxide and the RuII precatalyst). These interpretations are qualitatively in good agreement with preliminary DFT-based data.