[en] The L/sub I/-, L/sub II/-, L/sub III/-, and M-shell vacancy distributions accompanying electron shake-off in the α decay of ²¹⁰Po have been measured by α-x-ray two parameter coincidence techniques. The ionization probabilities and the errors due to our measurements were (5.11 plus-or-minus 0.40, 0.62 plus-or-minus 0.06, and 1.50 plus-or-minus 0.19) times 10^-4 for the L/sub I/, L/sub II/, and L/sub III/ subshells, respectively, and (1.8 plus-or-minus 0.4) times 10^-2 the M shell. Possible errors in tabulated fluorescence and Coster-Kronig yields could increase the uncertainties to (1.7, 0.5, 0.8) times 10^-4 and 0.6 times 10^-2, respectively. The energies and peak shapes of the α groups in coincidence with L/sub alpha/, L/sub beta/, L/sub gamma/, and M x rays were also measured. The coincident groups were, respectively, 12.6 plus-or-minus 1.4, 13.8 plus-or-minus 1.4, 17.6 plus-or-minus 2.1, and 3.1 plus-or-minus 1.0 keV lower in energy than the ²¹⁰Po α₀ peak. Theoretical calculations by Hansen are in closer agreement with the experimental values for the L subshells than Migdal's theory, but discrepancies of about a factor of 2 still exist