Belgian musical scholar who collected, described, and copied musical instruments (musical instrument) and wrote on acoustics and other subjects.

In 1865 Mahillon entered the instrument-manufacturing firm established by his father, Charles Mahillon. He also founded a music journal, L'Echo musical (1869–86). As curator of the Brussels Conservatoire museum (from 1879), he formed a collection of more than 1,500 ancient, modern, and non-Western instruments. His analytical catalog of the collection (1880–1922 in 5 vol.; reprinted 1978 in 2 vol.) contains demonstrations of theories of instrument construction and a classification of instruments based on the material that produces the sound (e.g., a drum is classified as a membranophone). This classification was later adopted and expanded by Erich von Hornbostel and Curt Sachs and has become the most commonly accepted system of instrument classification. He also made copies of rare instruments, notably the Bach trumpet, and organized concerts of music played on old instruments. Mahillon published Les Éléments d'acoustique musicale et instrumentale (1874; rev. ed. 1984; “Elements of Musical and Instrumental Acoustics”) as well as numerous monographs, and he also contributed articles to the ninth edition of the Encyclopædia Britannica.