Direct determination of the neutrino mass is at the present time one of the most important aims of experimental and theoretical research in nuclear and particle physics. A possible way of detection is through neutrinoless double-electron capture, 0νECEC. This process can only occur when the energy of the initial state matches precisely that of the final state. We present here a calculation of prefactors (PFs) and nuclear matrix elements (NMEs) within the framework of the microscopic interacting boson model (IBM-2) for 124Xe, 152Gd, 156Dy, 164Er, and 180W. From the PF and NME we calculate the expected half-lives and obtain results that are of the same order as those of 0νβ+β+ decay, but considerably longer than those of 0νβ−β− decay.