In the 1950s and 60s, McCrone conducted extensive research on the microscopic characterization of polymorphs, which he defined as materials that are "different in crystal structure but identical in the liquid or vapor states."  He investigated the difference in the properties of polymorphs of medications, co-authoring with John Haleblian a review article on "the pharmaceutical application of polymorphism", published in 1969.  McCrone's work on polymorphism exerted a strong influence upon the scientific career of Joel Bernstein.