Electron paramagnetic resonance (EPR) measurements have been made on HfO2 and ZrO2 both as films on silicon and as monoclinic phase powders with the aim of characterizing the defects in these high k oxides. The measurements on as-received, gamma irradiated, and both vacuum and air annealed powders reveal many types of defect, some of which are involved in charge trapping. Several, possibly all, of these defects may be at or near a surface of the particles. Films were deposited by e-beam evaporation but only those grown during low-energy Ar+ ion irradiation reveal EPR spectra of defects associated with the films. (C) 2009 American Vacuum Society. [DOI: 10.1116/1.3025882]