  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, $0\nu ECEC$. 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 (PF) and nuclear matrix elements (NME) within the
framework of the microscopic interacting boson model (IBM-2) for $^{124}$Xe,
$^{152}$Gd, $^{156}$Dy, $^{164}$Er, and $^{180}$W. From PF and NME we calculate
the expected half-lives and obtain results that are of the same order as those
of $ 0\nu \beta^+\beta^+$ decay, but considerably longer than those of $0\nu
\beta^-\beta^-$ decay.
