The study of the dielectric constant and loss in pure single-crystal MgO and in Fe- and Cr-doped crystals, the low frequency results of which have been reported [1], has now been extended to cover frequencies from 500 Hz to 9 GHz. Measurements were made on the same specimens, at room temperature, using bridge, cavity, slotted-line and cavity-resonator techniques. Over the whole frequency range, a good fit is obtained to the Universal Laws of dielectric response: σ ac ∝ ω n and (e′−e∞) ∝ ω n−1 with n=0.98±0.02 for both pure and doped MgO. At any point in this frequency range the addition of iron or chromium increases σ and decreases e′, the changes being more pronounced with iron. These effects are discussed in terms of hopping mechanisms.