A comparison has been made of the ‘relative impedance’ and ‘transmission’ techniques of measuring the small-signal parameters of varactor diodes. Relative-impedance measurements have been made at u.h.f. and X-band and transmission measurements at X-band. It has been shown that losses associated with the matching circuit used in relative-impedance measurements can give rise to a degraded diode specification. Their effect is less if reduced-height waveguide is used at X-band. The transmission method leads to the most favourable specification for a diode. The measurements support the representation of the diode by a simple equivalent circuit to a first order