The paper discusses selected problems of the complicated process of Serbian and Croatian language standardization during “national revival” in contrast to Slavists’ ideas. The newly founded discipline, Slavic philology, launched research into “language” understood as an indicator of national differentiation which influences other cultural and national factors. This paper supports a thesis that language was one of the most significant factors of national identification for the Croats and Serbs, and it analyses the most important moments in this process. In the description of South Slavic languages, Slavists were aware of a huge and essential linguistic diversity, but Slavistic classifications included mainly Serbian language. Another point presented in this paper is the description of Serbian and Croatian activity aimed at researching their national language and popularizing it, therefore the paper focuses mainly on the specific features of Lj. Gaj’s and Vuk Karadžić’s ideas concerning language. The paper concludes with an observation that linguistic criterion, originally constituting a factor of nation differentiation, for the Croats and Serbs gradually became− under mistaken belief that linguistic similarity is reflected in the similarity of culture − the main component of community projects that appeared in the nineteenth century among the South Slavs.