According to parallel distributed processing (PDP) models of visual word recognition, the speed of semantic coding is modulated by the nature of the orthographic-to-semantic mappings. Consistent with this idea, an ambiguity disadvantage and a relatedness-of-meaning (ROM) advantage have been reported in some word recognition tasks in which semantic processing is presumed to be required for responding. To further evaluate this idea, we examined ambiguity and ROM effects in lexical decision and semantic categorization tasks. In the lexical decision task, there was an ambiguity advantage but no ROM effect. In the semantic categorization tasks, we used various semantic categories and found a processing time disadvantage for ambiguous words with less related meanings when the decisions were relatively difficult, but observed no effect when the decisions were easier. These results suggest that both the ambiguity disadvantage and the ROM advantage in the semantic categorization tasks are due to decision-making, rather than semantic-coding, processes. The implications of these results for issues concerning the nature of semantic coding and semantic representations are discussed.