Abstract: The research subject is the creative work of a French composer, pianist, linguist, writer and philosopher Charles-Valentin Alkan (1813 - 1888). As the composer was interested in the legacy of the past - antique culture and Jewish folklore, he often used modal tonalities. Alkan was the first representative of academic music of the 19th century who used pentatonic scale in two etudes op.35 - in the F-major Allegro barbaro (op.35 no. 5), notably, in Lydian mode, and the B-major op.35 no. 6 based on pentatonic scale with an interesting comparison of natural and harmonical B-major. The author concludes that Alkan was the largest novator of harmonic language and mode peculiarities. He was one of the first musicians of the 19th century to use modal tonalities and colourful harmonies; he rearranged modes in the cycles of short scores, preludes, etudes and songs without lyrics, and he also was a novator in sonata cycles. To analyze Alkan’s works, the author uses comparative-analytical and comparative methods - Alkan’s music is compared with the music of his contemporaries, followers and the old masters whose influence on Alkan’s creative work is easy to see. The author also uses the music-analytical method, as it is very important to carry out the analysis - both merely melodic and harmonic - of each work of the composer. The scientific novelty of the research consists in the fact that the creative work of Charles-Valentin Alkan hasn’t been studied sufficiently enough in Russia so far. The only comprehensive thesis analyzing most Alkan’s works is the art history PhD thesis by Veniamin Smotrov “Charles-Valentin Alkan: personality, esthetics, creative work” having 415 pages. The problem of harmonic language hasn’t been studied in such a detail even in Smotrov’s dissertation as it is in this article. The author notes numerous peculiarities of harmonic language and modes and tonalities innovating in the creative work of Alkan and even some connections with synagogal Jewish symbolism.