[en] In a semiconductor with a large dielectric constant and a small electron effective mass, a positive ion and an electron can form a series of very weakly bound, macroscopic-sized shallow states. We present electric field (EF)-μSR measurements on CdS, where the final muonium state has an extremely low hyperfine coupling. In strong fields (higher than E_char∼5-10 kV/cm) the muonium signal is suppressed completely. We argue that muonium formation in CdS proceeds through the intermediate shallow states mentioned above. The experimental technique of EF-μSR measurements (using pulsed muon beams at ISIS) is discussed in detail