A series of molybdenum(vi) cis-dioxo complexes containing tetradentate salan ligands with different para-substitutions on the phenoxy group have been prepared. These complexes catalyse the oxygen atom transfer reaction between dimethylsulfoxide and triphenylphosphine. During oxo transfer catalysis, the complexes are resistant to formation of catalytically inactive oxo-bridged dimeric Mo(v) complexes. Electronic effects influence the rate of the oxo transfer reaction and the fastest rates are achieved when the para-phenoxy substituent is an electron withdrawing nitro substituent. Hammett correlations have shown that the rate-determining step involves nucleophillic attack of the phosphine on one of the oxo ligands. Electrochemical measurements have shown that all complexes containing tertiary amine ligands exhibit quasi-reversible behaviour and that the para-substituent has a considerable effect on the half potentials (E(1/2)). A linear correlation between the E(1/2) values and the Hammett sigma(p) parameter is observed.