Loeffler became a U.S. citizen in 1887 and eventually resigned from the orchestra to devote himself to composition. He was a friend of Eugène Ysaÿe, Dennis Miller Bunker, and John Singer Sargent (who painted his portrait), also of Gabriel Fauré and Ferruccio Busoni (both of whom dedicated works to him), and later of George Gershwin. A man of wide culture and refined taste, he developed an idiom deeply influenced by contemporary French and Russian music, in the traditions of César Franck, Ernest Chausson and Claude Debussy, and also by Symbolist and "decadent" literature. Loeffler often cultivated unusual combinations of instruments, and was one of the earliest modern enthusiasts for the viola d'amore, which he discovered in 1894 and wrote parts for in several scores as well as arranging much music for it. In his later years he also, unexpectedly, became deeply interested in jazz, and wrote some works for jazz band.