ABSTRACT. The relative permittivity and loss of zon e-refi ned si ngle crys ta ls of hexagonal ice have been measured in the temperature range 200-271 K a nd freq u e ncy range 0.5 Hz-O.2 MHz, using brass, stainless steel, and gold-foil electrodes. The c-axis of the crystal was oriented parallel to the electric field in 14 samples a nd perpendicular to the field in 8 sampl es. The eq uilibrium relative permittivity of orientation polarization '0, parallel and perpendicula r to the c-axis, is 96.5 ± I and the average relaxation time Tav is 36 I-'s at 265.5 ± 0.5 K ; '0 = 124 ± 1.5 a nd Tav = 30 ms, at 210 K. The magnitude of the orientatio n polarization obeys the Curie-Weiss equation with To = 15 ± 2 K fo r both the orienta tions. These values are in contrast with the c. 17 % difference in '0 for the two orientatio ns reported in the literature. The extrapolated limiting high-freq uency relative permittivity ' 00, measured fo r both the ori entations, is indistinguishable within 0.5 % .The loga rithmi c plot of the product of Tav a nd tempera ture against the reciprocal temperature is linea r in the range 210-271 K and gives an activation energy a nd a pre-exponential factor of 5 1 ± 2 kJ mol-I a nd 0.93 ± 0.22 ps K respectively, for both the orientations of the c-axis with respect to the electric field. The decrease in ac ti vatio n energy which h as been reported to occur in polycrystalline ice and in single crystal ice nea r 230 K is not found until a temperature of 2 1 0 K . Single crys tals of ice stored in the dielectric cell, after the completion of measurements, fo r periods ranging from 1-11 weeks at 253 ± 2 K showed no cha nge in their '0, Tav , ' 00 that could be attributed to the effect of ageing on the o ri e ntation polarization . R ESUME. Le trace logarithmique du produit Tav T, en fon ction de I'inverse d e la tem perature, I/ T , est linea ire dans le domain e d e temperature 210-27 1 K. On t ro u ve ainsi une energie d'activation de 5 1 ± 2 kJ mol-I et un fa cteur preexponentiel de 0,93 ± 0,22 ps K pour les d e ux orientatio ns de I'axe c par rapport a u champ electrique. La d ecroissance de I'en ergi e d'activation au.x a lentours de 230 K, qui a ete signaIee a ussi bien dans la glace polycristallin e que dans le glace monoc ristallin e n'a pu etre observee meme jusqu'a 2 10 K .Des mesures complt~tes de ' 0, Tav, ' 00 sur des monoc"istaux de glace co nser ves dans la cellule dielectrique pendant une p eriode de I it II semaines it 253 ± 2 K montrent que ces vale urs restent in changees; a u trement dit, il n'y a pas e u d'influence du vieillissement quant a u rol e de I'orientation de la polarisa tion. Der Betrag d er Orie ntierungspolarisation folgt einer Curie-Weiss-G leichung mit To = 15 ± 2 K fur beid e Kristallrichtunge n . Diese W erte stehen im Gegensatz zu d en etwa 17 % Unterschied in '0 fur die beiden Kristallrichtungen, die in der Lite ratur a ngegeben werden. Der extrapolierte Hochfrequenzgenzwert der relativen P ermittivitat, ' 00, gemessen in beiden Kristallrichtungen, ist innerh...