Abstract: The article is devoted to the phenomenon of substitution of terminological units based on the case study of nanotechnological terms in modern English and Spanish languages. The authors have chosen to study this topic as a result of a growing role of nanotechnology and a need in an adequate terminological layer and linguistic research. In the course of their research the authors analyze whether it is possible to present a specialized nanotechnological term both by terminological and common means, i.e. substitute terms in texts and discourses. The research is based on English and Spanish nanotechnological terms (2000 lexical units, 1000 lexical unit per each language). The terms were selected from nanotechnological research articles, journals, books, and Internet resources using the continuous sampling method and so were 1000 discursive fragments where selected terms had been used. The research methodology included the continuous sampling method that was used to select terminological units and discursive fragments, methods of discursive, contextual and content analysis, methods of component and definition analysis that allowed to anayze the phenomenon of the substitution of terminological units in texts and discourses. The authors conclude that in nanotechnology terms are substituted mainly on the terminological basis, i.e. terms are replaced by similar terms with a different morphological-syntactic structure or, more rarely, semantics. Within the framework of the analysis of English and Spanish nanotechnological terms, the authors define particular types of term substitution, formal-semantic and formal, the latter with the prevailing number of structural, morphological-syntactic, syntactic and hyper-hypernomic substitutes. They also define key reasons of the substitution of terms in texts and discourses.