This study examines the effects of teaching common complex grapheme-to-phoneme correspondences (GPCs) on reading and reading motivation for at-risk readers using a randomised control trial design with taught intervention and control conditions. One reading programme taught children complex GPCs ordered by their frequency of occurrence in children's texts (a "simplicity principle"). The other reading programme taught children word usage. Thirty-eight students participated in the 9-week programme of 30 supplemental small group sessions. Participants in the complex GPC group performed significantly better at post-tests with generally large value-added effect sizes (Cohen's d) at both by-participant and by-item for spelling, d?=?1.85, d?=?1.16; word recognition with words containing taught GPCs, d?=?0.96, d?=?0.95; word recognition, d?=?0.79, d?=?0.61, and reading motivation, d?=?0.34, d?=?0.56. These findings suggest that the simplicity principle aids in structuring maximally effective supplemental phonic interventions.