The article suggests a solution to the problem of the sister languages’ vocabulary comparison. The subject of research is Czech and Slovak vocabulary given in small bilingual dictionaries. The same metalanguage (Russian) of the definitions in such dictionaries makes the semantic analysis of vocabulary easy. The basic method is parametric analysis. It implies the ranking of words of a language according to four system parameters: functional, syntagmatic, paradigmatic, and epidigmatic ones. For each parameter, it is necessary to distinguish a local nucleus, which includes about 1000 words. Local parametric vocabulary nuclei make it possible to identify (1) the shortest, i.e., non-derivative and common (frequently used) vocabulary in these Slavonic languages; (2) vocabulary most commonly used in various fixed expressions and collocations; (3) vocabulary that forms the largest synonymic chains; (4) the most polysemantic vocabulary. The addition of local nuclei allows forming a big parametric nucleus. It includes words, which possess parametric weight values based on at least three parameters (out of four). For Czech, the big parametric nucleus comprises 279 words. For Slovak, it comprises 282 words. These lexical sets present the subject of the further analysis aimed at classifying the meanings of the words. At first, the authors establish that Czech and Slovak nuclei contain 61 words meaning the same in both languages. Besides, among them, there are 30 words that possess not only the same meaning but the same form as well. The big parametric nuclei do not contain false friends, i.e. words that look similar but differ in meaning. Then the authors do qualitative analysis of nuclear lexemes’ semantics. They find out that, in Czech and Slovak, basic semantic groups are formally similar, but they differ in intragroup distribution. Language data illustrate the fact that such semantic groups as “Activity”, “Abstract Notions”, “Human Being” are the dominant semantic groups in both languages. In the Slovak big parametric nucleus, a quantitative gap between them is less perceptible than in the Czech one. Besides, the semantic group “Human Being” occupies a higher position in the Slovak parametric nucleus. The dominance of the semantic group “Activity” and of the big parametric nucleus, as well as a negligible percent of words in the subgroup “Imitating Activity”, justifies the fact that, for Czech native speakers, not a person but a person-agent is of a particular importance. At the same time, the high percents of the semantic group “Nature” and of subgroup “A Person of Nature” prove that great attention to nature objects is a feature of Slovak native speakers’ worldview.