  We analyse the sensitivity of geosynchrotron radio emission from inclined
extensive air showers to the energy and mass of primary cosmic rays. We
demonstrate that radio emission measurements at suitable lateral distances can
infer both the number of electrons and positrons in the shower maximum and the
atmospheric depth of the maximum on a shower-to-shower basis. Alternatively,
measurements at a fixed lateral distance but in two different observing
frequency bands yield comparable information. An RMS error of 5% in the
determination of the number of electrons and positrons at shower maximum can be
achieved. Through the determination of these quantities, geosynchrotron
radiation provides access to the energy and mass of primary cosmic rays on a
shower-to-shower basis.
