The electrochemical behaviour of hydrides [CoHL₄][L = P(OEt)₃, PPh(OEt)₂, or PPh₂(OEt)] in CH₂Cl₂ and MeCN was examined and the relevant oxidation mechanisms elucidated by cyclic voltammetry and coulometry. Different behaviours were observed depending on the nature both of the solvent and of the phosphite ligands. In CH₂Cl₂ the stepwise oxidation of Co^I to Co^III was observed, involving a first reversible one-electron oxidation step to [CoHL₄]⁺, followed, at more positive potentials, by a second one-electron step, chemically irreversible, for all the phosphite ligands employed. In MeCN, for L = P(OEt)₃ or PPh(OEt)₂, the first reversible oxidation to cobalt(II) hydrides was followed by a second step complicated by associated chemical reactions involving the solvent to give cobalt(III) cations, [CoH(MeCN)L₄]²⁺. For L = PPh₂(OEt) the electrochemical behaviour was complicated by chemical reactions both of the starting cobalt(I) compound and of its oxidation products with the MeCN solvent. Preparative electrolysis allowed the synthesis of a new cobalt(II) derivative characterized as [CoH{P(OEt)₃}]BPh₄ by spectroscopic techniques.