The theory and operation of an equipment developed for laboratory use in the frequency range 50–500 Mc/s is described. The principle of the equipment is to compare the modulus of any impedance with that of a standard impedance, namely a 100-ohm 0.1-watt resistor mounted so as to keep the lead length constant and at a minimum.From four values of impedance moduli determined experimentally the resistive and reactive components of the unknown can be calculated.This generalized method of determining an unknown impedance can be simplified for particular, and important, cases, such as the measurement of high- and medium-value resistors, inter-electrode impedances of valves, inductances, capacitances. Impedances in the range 1–100000 ohms can be determined. The error in the measurement of the modulus of an unknown impedance is less than ±5%, whilst that of the phase angle varies with the value of impedance being measured. Examples of measurements made on resistors, crystal valves, and short-circuited coaxial transmission lines are given.