German-born language scholar who made the first major analysis of the Romance languages and thus founded an important branch of comparative linguistics.

As a student Diez acquired a deep interest in Italian poetry, but a visit to the great German poet J.W. von Goethe in 1818 directed his attention to exploring Provençal literature. As Privatdozent, or student-paid lecturer, at the University of Bonn from 1822, he published two important early works, one on Provençal troubadour poetry (1826) and the other on the lives and works of the troubadours (1829). After becoming professor of modern literature at Bonn in 1830, he turned increasingly to more general considerations of the Romance languages. Thus he came to produce his two great works, Grammatik der romanischen Sprachen, 3 vol. (1836–44; “Grammar of the Romance Languages”), and Etymologisches Wörterbuch der romanischen Sprachen, 2 vol. (1853; “Etymological Dictionary of the Romance Languages”). His work in the area of the Romance languages has been compared with the great achievement of Jacob Grimm in the field of Germanic languages.