AbslncL m i s paper reports the mulls of a vplematic study of the 9.355 GHz mmplex permittivity of doubledislilled, deionized water over the temperalure interval from I s ) lo Ws) in -2 s ) steps. A precision of -0.03% and an accuracy of -0.1% -re achieved lor both lhe permitlinty 6 ' and Ihe dielectric lw e": r'(T) = 43.789(18) + 1.4098(18)T -3.008(26) x 10-z'Tz + 2.249(71) x IO-%"' -2.33(8.5) x lO-'~-3.14(36) x 10-9Ts; r"(T) = 39.9615(91)l.77(20) x 10-"7'-3.350(13) x 10-zTz+8.387(36) x 10-"T3-8.470(42) x 10-6T'+3.146(18) Y 10-8Ts where T is in T. It shows lhal the values of the thermodynamic pwameters envow (AS), enthalpy (AH), and h e energy (AF) of activation are temperature dependenl, mnlrary U) previous reportr The temperalure dependence of the relaxation time in this temperalure interval b in good agmmenl with thal reponed in the lileralure. ?he values of the thermodynamic functions, calculaled using the various pain of a and cm values reponed in Ihe litemlure. agree within lo over most of the measuremenl temperalure interval.

Recent improvements to a high-precision instrumentation system for measuring the complex permittivity of high loss liquids using a variable-length transmission sample cell at microwave frequencies are described. An error analysis for this system is given. These improvements have enabled the complex permittivity of heavy water and of double-distilled, deionized light water to be measured with a typical accuracy of &0.1% for 6' and f0.2% for 6'' and a precision of about 0.02% at 9.355 GHz in the temperature interval from approximately 1 to 90 "C in increments of about 2.5 "C. The values of ~' ( t ) and ~" ( t ) have been fitted to empirical quintic polynomials in the temperature t ("0. For light water ~' ( t ) = 44.628(3) + (13.929(8) x 10-l)t -(3.222(6) x 10-2)t2 + (3.165(17) x 10-4)t3 -(1.503(21) x + (2.67(9) x 10-9)t5 and ~" ( t ) = 40.573(3) -(1.475 (6) x 10-l)t -(2.477(4) x 10-2)t2 + (6.092(12) x 10-4)t3 -(6.000(15) x 10-6)t4 + (2.213(7) x 10-8)t5 and for heavy water ~' ( t ) = 31.452(17) + (16.630(28) x 10-l)t -(2.796(17) x 10-2)t2 + (1.141(45) x 10-4)t3 + (0.920(54) x 10-6)t4 -(7.16(23) x 10-9)t5 and ~" ( t ) = 37.610(9) + (4.952(16) x 10-1)t -(4.749(10) x 10-2)tZ + (9.659(26) x 10-4)t3 -(8.609(30) x 10-6)t4 + (2.913(13) x 10-8)t5. The temperature dependences of the Gibbs free energy, enthalpy, and entropy of activation parameters for the relaxation process as well as the relaxation time have been calculated from both the ~' ( t ) and ~" ( t ) data sets for both light and heavy water.