Inflection of Belarusian surnames is a topic of nationwide importance in Belarus. To this day, there is a great diversity in reference literature, and the standardization of surname inflection has not been established. The article describes the declension variations of Belarusian surnames in the Belarusian language, influenced by Russian patterns that are incompatible with standard practices of Belarusian speakers. Discussion includes the formation of the Belarusian linguistic norm, which has experienced historical and continuing influence by extra-linguistic factors. Complex instances of declension of Belarusian surnames in both Belarusian and Polish as well as of Polish surnames in both Polish and Belarusian are thoroughly discussed. On the one hand, comparative analysis shows the subtle similarities and differences between the two Slavic languages, from which interference errors can result. On the other hand, there are noticeable tendencies for 1) Belarusian surnames to decline according to the Russian models, as well as 2) Belarusians living in Poland to express preferences to decline Belarusian surnames used in the Polish language, according to the rules of Belarusian language.