Kinetic analysis of the successive oxidative cyclic voltammetric responses of [Os(II)(bpy)(2)py(OH(2))](2+) in buffered water, together with determination of H/D isotope effects, has allowed the determination of the mechanisms of the successive proton-coupled electron transfers that convert the Os(II)-aquo complex into the Os(III)-hydroxo complex and the later into the Os(IV)-oxo complex. The stepwise pathways prevail over the concerted pathway in the first case. However, very large concentrations of a base, such as acetate, trigger the beginning of a concerted reaction. The same trend appears, but to a much larger extent, when high local concentration of carboxylates are attached close to the Os complex. The Os(III)-hydroxo/Os(IV)-oxo couple is globally much slower and concerted pathways predominate over the stepwise pathways. Water is, however, not an appropriate proton acceptor in this respect. Other bases, such as citrate or phosphate, are instead quite effective for triggering concerted pathways. Here, we suggest factors causing these contrasting behaviors, providing a practical illustration of the prediction that concerted processes are an efficient way of avoiding high-energy intermediates. Observation of a strong decelerating effect of inactive ions together with the positive role of high local concentrations of carboxylates to initiate a concerted route underscores the variety of structural and medium factors that may operate to modulate and control the occurrence of concerted pathways. These demonstrations and analyses of the occurrence of concerted pathways in an aquo-hydroxo-oxo series are expected to serve as guidelines for studies in term of methodology and factor analysis.