Hyperfine structure; apparently of pure quadrupolar form; has been observed in the Eu 3+ ion in a neodymium ethylsulfate lattice. It has twice the predicted magnitude and opposite sign to that calculated on the basis of a pure ground electronic configuration of the type 4 f n . Arguments are presented for ascribing the anomaly to the admixing; through the crystal-field potential; of the state 5 p 5 6 p 1 D 2 into the closed shell state 5 p 6 1 S 0 . For Eu 152 ; we find P 152 =-(6.7±0.5)×10 -4 cm -1 and for Eu 154 ; P 154 =-(8.3±0.7)×10 -4 cm -1 . The spin and parity assignments of 2- for the 1531-keV state in Sm 152 and the 1400- and 1723-keV states in Gd 154 ; as well as the electric-dipole multipolarities of the radiations depopulating these states; are confirmed. The quadrupole moment of Eu 154 is found to be 3.29±0.37 b.