The K-shell auto-ionization probability has been measured by Ge(Li) spectroscopy for three β− decays of low, medium, and high energy. The values obtained, (0.041 ± 0.006) × 10−4 for 151Sm, (1.0 ± 0.2) × 10−4 for 169Er, and (1.23 ± 0.1) × 10−4 for 210Bi, are discussed in terms of existing theories, and modifications to the latter are suggested.