Employing a large sample of children from Dutch regular elementary schools, this study assessed the contributing and discriminating values of reading disability (RD) and attention-deficit/hyperactivity disorder (ADHD) to two types of phonological processing skills, phonemic awareness (PA) and rapid automatized naming (RAN). A second objective was to investigate whether comorbidity of RD and ADHD should be considered as an additive phenomenon as to RAN and PA. A total of 1,262 children, aged 8 to 13 years, were classified as RD (n = 121), ADHD (n = 17), comorbid (RD+ADHD; n = 16), or control (n = 1,108). Phonological processing was assessed by standardized tests of PA and RAN. Disability groups were compared to each other and contrasted to the control group. Although results indicate substantial effects for all three disability groups on both types of phonological processing, and the RAN/PA compound measure in particular, effect sizes were considerably larger for the RD groups, as compared to the ADHD-only group. Theoretical and practical implications are discussed.