The two-neutrino double,6 decay matrix elements of Ni-58, Ru-106, 106Cd, and Ce-136 are calculated in the framework of the quasiparticle random phase approximation and the corresponding double beta+/electron-capture (EC) decay (beta+beta+/beta+EC/ECEC) half-lives are estimated. The calculated ground-state-to-ground-state half-lives range from 3.9 X 10(23) to 6.4 X 10(19) yr indicating that the double beta+/EC decay could, in principle, be detected using currently known experimental techniques.