The mixed-metal single-cubane-type complex [Mo3CuS4(H2O)(10)](4+), which was generated by disintegration of the double-cubane-type complex [(H2O)(9)Mo3S4CuCuS4Mo3(H2O)(9)](8+), was studied by EPR spectroscopy and EHMO calculations. The hyperfine coupling constants of the molybdenum and copper nuclei were determined from frozen solution EPR spectra for the Mo-95-enriched and -unenriched complexes. EPR analyses show that the single-cubane-type complex [Mo3CuS4(H2O)(10)](4+) is a paramagnetic of S = 1/2 and the paramagnetic center is mainly situated on one of the three molybdenum ions; the three molybdenum ions are not equivalent to each other, but they are formally close to Mo(III), Mo(IV), and Mo(IV), respectively. It is also shown that there is little spin density on copper, which may be close to Cu(I) rather than Cu(II), and that the paramagnetic site, Mo(III), is jumping among the three molybdenum ions at such a rate as to cause a line width broadening effect on the molybdenum hyperfine components of the EPR spectrum.