The electrochemical oxidation of tris (2,4,6-tri-methyl-phenyl) phosphine (TMP) in acetonitrile was investigated on a platinum electrode. A fast mono-electronic transfer (E-p = 0.71 V vs sce) gives rise to the corresponding long-lived phosphoniumyl cation radical. This electrochemically generated radical undergoes a slow reaction with residual water, generating the corresponding phosphine oxide (TMPO) and hydro phosphonium (TMPH+). The kinetics of the decay of the cation radical has been studied, and a reaction mechanism is proposed. When TMP is oxidized on a platinum electrode at potentials higher than 1.5 V vs see, a fast surface reaction involving residual water takes place. This process was studied by voltammetry at a rotating ring-disk electrode; the same products, TMPO and TMPH+ were formed.