The dialectal system of noun formation often demonstrates much greater inflectional variability in a single case position than in the literary language. This feature of the dialect paradigm is determined by the consequences of the active interaction of ancient substantive stems and innovation at the present stage of form formation of word-change norms.
 The best way to trace the specific features of filling the paradigm in general and the degree of inflectional variability specifically in a particular link is to observe spontaneous dialect speech for a long time and to form a corpus of textual records based on fixed spoken testimonies. This approach to the description of the relevant linguistic phenomena provided the basis for realizing the goal of this study – to find out the specifics of the formation of the instrumental case in the plural of substantives in the Western Polissian and related dialects based on the most complete composition of dialectal manifestations of nouns; to comment on the factors influencing the existence of dialectal differences in the studied case position.
 The case study materials of the research are the authors' records of factual material made in 146 localities in the Volyn and Rivne regions.
 The presented and analyzed material gives grounds to assert that the formation of the instrumental case of the plural nouns in the dialectal language system, as well as in the literary one, is determined by the unification of inflectional types based on the model of the corresponding manifestations of the substantives, primarily ancient *-ā-, *-jā-stems, rarely – formal vocal *-ǐ-stems. The folk conversational tradition also indicates innovations that reflect the consequences of phonetic processes. Thus, inflectional variability in the analyzed case position is determined by differences in the realization of vowel and consonant sounds, typical to the sound system of the Western Polissian and related dialects. Among the archaic word-change phenomena in the studied range, the remnants of ancient dual forms of the instrumental case of substantives are attested.
 The article provides demonstrative examples of variant implementation of a particular ending in a particular subdialect, as well as inflectional variants of a case position of a particular noun, which may indicate that forming dialectal inflectional norms is still ongoing.