This article presents chosen formative phenomena which illustrate regional features of Stanislaw Moniuszko's language. His idiolect connected features of the general literature Polish and the Polish of developed stratum of north-east verges. The authoress has researched all 794 letters collected and published by the musicologist W. Rudzinski. The analysis of language material has demonstrated that there is no fundamental defection with relation to the Polish-wide system. The various diminutive, augmentative and hypocoristic formations present the most curious and numerous group. Such a big attendance of them is caused by the author's borderland origin. There are some properties which belong to the north-borderland word-formation, such as: occurrence of suffix '-uk (-czuk)', productiveness of suffix '-osc', productiveness of adjectival suffix '-(ow)ski' in substantival derivatives, fluctuations of adverbial suffixes '-e II -o' and fluctuations of verbal prefixes.