Infrared spectra of solid NaXO2(Ac)3, with X=U, Np, Pu, and Am, have been observed. From the symmetric and asymmetric stretching frequencies of the O-X-O groups, approximate X-O force constants have been calculated and were found to decrease in the order kU—O>kNp—O>kPu—O> kAm—O, the respective values being about 0.705, 0.698, 0.675, and 0.612 megadyne/cm. From the cell constants for NaXO2(Ac)3 it is apparent that the X-O bond distance decreases in the same order—RU—O>RNp—O>RPu—O>RAm—O. Thus, a decrease in bond distance appears to be accompanied by a decrease in force constant, probably because the bond, though shortened by contraction of the electron shells of the metal, is weakened by interaction with the extra valence shell electrons.