The following study deals with the structural elements of the six Mendelssohn Organ Sonatas, opus 65. The problem excludes stylistic considerations. The Mendelssohn organ works are the greatest that appeared from the time of J. S. Bach until the nineteenth century. The Sonatas "have long been accepted as 'classics' of the instrument." Of them Mendelssohn himself wrote in a letter to his publisher, "I attach much importance to these Sonatas." They have no formal predecessor and no formal counterpart in subsequent organ literature. Their structures are hybrid, contrasting, unique, and unconventional; yet, to the writer's knowledge, there is no material available which presents a thorough, scientific formal analysis. It is hoped that the following study will fill this need.