Finno-Ugric languages
Jaunius studied the relationship between Indo-European languages and Finno-Ugric languages or Semitic. He left notes for Lithuanian–Estonian (446 words) and Lithuanian–Finnish (474 words) etymological dictionaries. He also prepared a dictionary of loanwords of Baltic origin in the Finnish language (222 words). He likely became interested in Finno-Ugric languages after reading a work of August Ahlqvist in 1878. He claimed to have discovered equivalents of consonants in Proto-Indo-European and Proto-Semitic languages. In his last decade, Jaunius was interested in many different topics and started many projects, but was unable to finish them. A collection of his previously unpublished manuscripts was published in 1972.