A theoretical model is presented for the coupled evolution of cerium and neodymium isotopes in terrestrial rocks. Data presented for samples ranging from mid-ocean-ridge basalts to continental metamorphic rocks show that, while mantle-derived rocks are dispersed on a Ce-Nd diagram, continental crustal rocks form a clearly defined 'mantle array'. As analytical techniques improve, cerium isotopes will provide valuable constraints in studies of petrogenesis and crustal evolution.