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The calibration of modern automatic RLC bridges requires a set of impedance standards. Each standard needs in turn a calibration, i.e. the knowledge of its four-terminal-pair impedance Z4TP at frequencies up to 10–100 MHz—frequencies beyond the capabilities of standard metrological setups for impedance traceability (transformer bridges). It is possible to think of an impedance standard as a multiport linear radiofrequency network; the properties of the network can be mathematically related to its Z4TP. In the literature, such a connection is made using the relation between the impedance matrix Zij and Z4TP, and with careful identification of an ad hoc lumped-parameter electrical model of the standard. A vector network analyser is employed to measure diagonal elements of Zij directly; non-diagonal elements are measured in an indirect way. Here we derive a new relation between the scattering parameter matrix sij and Z4TP of the impedance standard. All sij elements can be directly measured with a network analyser: we measure scattering parameter spectra of a set of commercial gas-dielectric capacitors. Computed Z4TP spectra are compatible with those obtained with the Zij technique, but the new method proves much more sensitive, permitting estimates of the frequency behaviour of an impedance standard even when electrical modelling is inconvenient or impossible.