  According to recent experimental data at GSI, the rate of the number of
daughter ions 140Ce58+, produced by the nuclear K-shell electron capture (EC)
decay of the H-like ion 140Pr58+, is modulated in time with a period T_d =
7.06(8) sec and an amplitude a_(EC) = 0.20(2). We show that this phenomenon can
be explained by neutrino-flavour mixing and derive a value for the difference
of squared masses Delta m^2_(21) = m^2_2 - m^2_1 = 2.22(3)x10^(-4) eV^2.
