Louis Vierne indicated his 24 Pièces en style libre as being for organ or harmonium and he provided separate registrations for each instrument. Nevertheless, several organists and authors have noted a disparity in the work's success on the two different instruments. This study begins with a look into the development of the harmonium and a consideration of some important traits of standard harmonium literature as exemplified by Franck and Guilmant. An examination of Vierne's pieces from this harmonium perspective reveals several problems, mostly in the areas of compositional texture, the harmonium's air supply, and Vierne's harmonium registrations, which often point in opposite directions from his verbal indications in the score and his organ registrations. This study concludes that Vierne's 24 Pièces are genuine organ music and that Vierne made little attempt to conform to the musical capabilities and constraints of the harmonium, even as he hoped that indicating the harmonium would increase the market appeal of this volume.