The incident, reflected and transmitted powers were measured and the reflection and transmission coefficients calculated for a liquid fuel. Two sizes of double ridge waveguide were used to cover the frequency ranges 2.5 to 7.5 and 7.5 to 18 GHz and corrections were made for waveguide losses. Strong interference effects were observed and the real part of the dielectric constant, ε', was obtained primarily from the separation of interference maxima and minima while the loss tangent, ε“/ε', was obtained by curve fitting to the normalized power loss. The final values of ε′ and ε“/ε′ were determined at each frequency by minimizing the difference between the experimental and theoretical values of the reflection coefficient and the normalized power loss. With increasing frequency ε' decreases and then becomes constant while ε“/ε' decreases throughout the frequency range within experimental accuracy.