Conduction electron spin resonance (CESR) has been measured between room temperature and 4 K on small caesium metallic clusters produced in zeolite X. The g value is 1.970ŠŷŠ0.005, in reasonable agreement with theory. The signal intensity exhibits a Curie-Weiss temperature dependence because the clusters are so small that quantum size effects are important. However, the latter have a much smaller effect on the g value than the large spinorbit coupling. © IOP Publishing Ltd.