Electron-paramagnetic-resonance experiments on Gd3+ impurities in the singlet-ground-state zircon systems EuAsO4 and EuVO4 have been used to study the various possible dynamical interactions in these systems. The low-lying degenerate excited states of Eu3+ in these compounds can produce dynamical electric, magnetic, and exchange fields in the lattice. From the ratios of the linewidths in the fine-structure spectra of EuAsO4(Gd) and EuVO4(Gd), we find that the dynamical magnetic and/or exchange fields are dominant over the dynamical electric fields caused by the Jahn-Teller effect. From the relative magnitudes of the linewidths in the two compounds, we conclude that exchange interactions are more important than the magnetic dipolar effects. © 1979 The American Physical Society.