We evaluated the impact of teaching complex grapheme-phoneme correspondences (GPC) derived from the Simplicity Principle to at-risk poor readers in Grade 2 classrooms, using a two-arm dual site matched control trial intervention. Poor word readers (n = 149) were allocated to either a) Simplicity GPC (n= 79) or b) Letter-Name Control (n= 70) small group reading programs, and received intervention for 12-15 hours over 12 weeks. Students were matched on baseline reading, language, parent demographics, and observed regular classroom teaching quality. Results of hierarchical data modeling showed advantages for the GPC-group for word reading, pseudoword reading, and sentence comprehension at post-test moderated by pre-test phonological awareness skills. The results provide support for teaching complex GPCs derived from the Simplicity Principle as an approach to intervention for word reading, but suggest that children with low PA need additional supports.