The second-order WKB quantization rule is applied to the determination of the eigenvalues associated with electron thermalization in gases. It is shown that a few percent error in the WKB eigenvalues can be reduced by the second-order correction to less than 0.1% in the case of the electron thermalization in hard-sphere and helium gases with zero electric field. A large error of −116% is drastically reduced to 5% for the lowest eigenvalue associated with electron attachment to SF 6 in He. The usefulness of the second-order WKB correction is thus confirmed.