The present work gives the results of measurements of the temperature variation of the dielectric constant, loss angle, and conductivity, of a sample of chemically pure glycolphthalate resin. The effects of known amounts of impurity are investigated. It is shown that the conductivity obeys accurately an inverse exponential law of temperature. The dielectric constant and loss angle follow qualitatively, but not quantitatively, the Debye equations for a polar material. The cause of this discrepancy is discussed, and a resume given of the behaviour of other types of polar material, of which six classes seem to be distinguishable.A brief indication is given of seven possible modifications of the Debye theory, not involving a distribution of relaxation times, which have been investigated and shown to be unsatisfactory. It is concluded that a distribution of relaxation times is, of those considered, the only possible hypothesis consistent with observation, and a quantitative expression for the loss angle is worked out and shown to agree well with observed data. The results show that the viscosity to dipole rotation follows the same law of temperature variation as the d.c. resistivity.Appendices give the method of preparation of the chemically pure resin; the circuit of a 50-kc/sec. bridge which has been found simple and convenient for this work; and the computation of an integral which occurs in the prediction of the loss angle and seems not to have been previously computed.