This study compares the semantic networks of the verbal prefix u- in two South Slavic languages, Bulgarian (blg.) and Croatian (cro.), in a cognitive linguistics framework using two databases of prefixed verbs drawn from dictionaries and corpora. We point to similarities and overlapping categories in the semantic networks of u- in blg. and cro., as well as to differences. When accounting for the differences, we consider prefixes semantically similar to u- (within one language and in both languages) that combine with the same base verbs forming either prefixed near-synonyms (e.g., blg. uvehna, zavehna, izvehna ‘wither, fade’; cro. umastiti, omastiti, zamastiti ‘grease, stain’) or prefixed verbs that are only seemingly near-synonyms (e.g., blg. ukaža ‘indicate, show’, okaža se ‘turn out’; cro. ocrniti ‘denigrate, slander’, ucrniti (se) ‘color black, wear black’). We examine how the meaning networks of individual semantically related prefixes (e.g., u- and za-, iz-) overlap within one language and across the two languages, as well as how different construals of the same event affect the prefix choice.