The article offers a cross-linguistic study of the English space prepositions in, on and at and their counterparts in Armenian. The choice of these prepositions for analysis is predetermined, firstly, by the fact that they are often used arbitrarily in English, when the priority of one over another in a particular prepositional phrase appears rather obscure and not motivated. Secondly, English space prepositions are rendered into Armenian by means of both postpositions and case inflections, with the latter functioning as morphological synonyms to the former. The analysis is carried out in the framework of cognitive semantics as well as the theory of oppositions which help distinguish all the subtle differences in the usage of these linguistic units triggered in most cases by extralinguistic factors. The study reveals that the oppositions made up with the space prepositions in, on and at in English outnumber those in Armenian and that their functional scope in English is rather extensive and diversified in comparison with those in Armenian.