The thesis covers sociolinguistic and psycholinguistic aspects of Czech and Slovak language relations. To both fields, it contributes in the form of own empirical research. At the centre of attention are native speakers of Slovak living long-term in Czechia and their language behaviour under this setting in relation to Czech and Slovak. Given that two languages in question are genetically very close and mutual intelligible, the actual language behaviour of the described population is not strictly predetermined by the social norms (Dickins, 2009). However, there are also conditions supporting the accommodation to Czech (Sloboda, 2005). The thesis focuses on the identification of factors influencing language choice and on the impacts of such factors on the psychlinguistic processing of Slovak and Czech words. The sociolinguistic part builds on questionnaire-based surveys, such as those conducted by Sloboda (2006). The own questionnaire survey was carried out via web. The data from 651 respondents were assessed, all from native speakers of Slovak having grown up in Slovakia and commencing their stay in Czechia after the age of 18. The data revealed that Slovak is used more than Czech, that Czech is more often perceived, that the use of Czech is more common in the communication with strangers or in...