THE mechanism of superconductivity in alkali-metal compounds of C 6o (refs 1, 2) remains controversial. Electron-pairing mechanisms based on electron-phonon coupling involving both low-frequency (intermolecular) vibrations3 and high-frequency (intramolecular) modes4,5 - the strong-coupling and weak-coupling cases respectively - have been proposed. These two mechanisms have different associated energy scales, which is reflected in the magnitude of the superconducting energy gap. Measurements of the gap for these compounds have been reported previously for powder samples6-8, but are somewhat discrepant or ambiguous, perhaps owing to grain-size or grain-junction effects. Here we report measurements on large single-crystal samples of K3C60 and Rb3C60 using optical reflectivity. We obtain values for the reduced gap ratio, 2Δ/k8Tc of 3.44 and 3.45 respectively, consistent with predictions for a mechanism based on standard Bardeen-Cooper-Schrieffer (BCS) electron-phonon coupling to intramolecular modes.