Describes a new instrumentation system which enables the complex permittivity of high loss liquids to be measured with improved precision at microwave frequencies. This system has been realised at 9 GHz and it has been possible to make an absolute determination of the complex permittivity of water to an accuracy of 0.06% in epsilon ' and 0.15% in epsilon " compared with 0.5% and 1% previously. It also permits differential measurements to be made such that 0.001 M/1 rather than 0.5 M/1 KCl can be detected at 9 GHz. This instrumentation system features (i) microwave power source and local oscillator with 4:10¹⁰/0.1 s spectral purity and stability, (ii) double superheterodyne synchronous demodulation at 60 MHz and 1 kHz, (iii) precision 1 kHz attenuation and phase measurement with a resolution of +or-0.01 dB and +or-0.01 degrees , respectively.