Muon spin rotation experiments have been performed on filled skutterudites PrOs4Sb12 and REPt4Ge12 (RE = La, Pr, Nd, Eu) over a temperature range 0.025–300 K in magnetic fields up to 7 T. An electron bound state is detected spectroscopically in filled skutterudites containing magnetic ions, both below and above the superconducting transition temperature Tc and both below and above the corresponding upper critical magnetic field Hc2, including the pseudogap region. This state is proposed to be a spin-singlet —an electron pair confined around the positive muon.