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 epsilon’ and +/-0.2% for epsilon" and a precision of about 0.02% at 9.355 GHz in the temperature interval from approximately 1 to 90-degrees-C in increments of about 2.5-degrees-C. The values of epsilon’(t) and epsilon"(t) have been fitted to empirical quintic polynomials in the temperature t(degrees-C). For light water epsilon’(t) = 44.628(3) + (13.929(8) x 10(-1)t - (3.222(6) x 10(-2)t2 + (3.165(17) x 10(-4)t3 - (1.503(21) x 10(-6)t4 + (2.67(9) x 10(-9)t5 and epsilon"(t) = 40.573(3) - (1.475(6) x 10(-1)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 epsilon’(t) = 31.452(17) + (16.630(28) x 10(-1)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 epsilon"(t) = 37.610(9) + (4.952(16) x 10(-1)t - (4.749(10) x 10(-2)t2 + (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 epsilon’(t) and epsilon"(t) data sets for both light and heavy water.