A comparative study has been performed of hydrogen and deuterium sorption inside a thin layer palladium electrode from acidic and basic solutions containing lithium cations. Lithium presumed to be incorporated in electrode, has an influence on the alpha- to beta-phase transition only in basic solutions, as a result of more negative potentials values of hydrogen (deuterium) absorption. An isotopic effect has been observed in basic solutions during of absorbed deuterium due to interaction mostly between alpha phase of absorbed hydrogen and incorporated lithium