The compound Sr-2(Sc1+xNb1-x)O6-x has the ABO(3) perovskite structure. It becomes non-stoichiometric by changing the ratio of Sc3+ to Nb5+ ions, i.e. by varying x. For x > 0, this compound is an oxygen ionic conductor, but it becomes an excellent protonic conductor, with activation energy 0.63 eV, after pretreatment in water vapor. The protonic conductivity is shown to vary proportionately to x, characteristic of Stage II (unassociated) behavior. In the case of samples with x less than or equal to 0, the compound does not take up hydrogen, and, in fact, remains an oxygen ionic conductor even though the oxygen-vacancy concentration is suppressed in the case of negative x. There is no evidence for conduction by electronic defects or by A- or B-site vacancies.