Two Ru^IV=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 [Ru^II(L¹)(solvent)]²⁺/iodosyl benzene system has an initial TOF of approx. 40 h^-1 and quantitative yield, with [Ru^II(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 Ru^II 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, Ru^II-based isomer, and inefficient ligand exchange in the product (hydrolysis produces the free sulfoxide and the Ru^II precatalyst). These interpretations are qualitatively in good agreement with preliminary DFT-based data.