Sergei Nikolaev is a specialist in Slavic and Indo-European comparative historical linguistics. The scope of studies covers Slavic,
Balto-Slavic and Indo-European historical
accentology, comparative
grammar of
North Caucasian languages, hypothetical
Sino-Caucasian macro-family of languages, hypothetical
Amerindian macro-family. Nikolaev connects the
linguogeography and historical dialectology of Slavic languages with the problems of . In
East Slavic dialectology, he established a number of the oldest (Late Proto-Slavic) dialect
isoglosses. Reconstruction of the placement of these isoglossae on the territory of the oldest Slavic settlement showed their connection with the archaeological areas of large Proto-Slavic tribal associations. As a result of the field studies of the East Slavic dialects, the Institute has collected an East Slavic phonetic library and an archive of
dialect recordings. Nikolaev is the author of the etymological dictionary of
North Caucasian languages (together with
Sergei Starostin), one of the authors of the series "Fundamentals of Slavic accentology", a participant in the international Internet projects "Evolution of language" and "tower of Babel" (etymological databases; he created a comparative database on Indo-European languages and databases on
Finno-Ugric and Amerindian languages). For more than 20 years, Nikolaev has been the leader and organizer of complex linguistic expeditions to the East Slavic dialects (the Carpathians, the Russian Northwest,
Belarus,
Polesie) and to the archaic Old
Shtokavian dialects of the
Serbo-Croatian language (
Slavonia, together with Croatian colleagues), author of special field programs on East Slavic historical dialectology. Nikolaev was nominated three times as a
Corresponding Member of the
Russian Academy of Sciences. In 2004, he was awarded the Sigma Xi, The Scientific Research Society (
Montreal,
Canada). In 2014, after placing the accents, Nikolaev confirmed (with a significant transformation and many clarifications) 's idea of syncretic polyrhythm in ''
The Tale of Igor's Campaign''. ==Contribution to the study of butterflies==