The present study was designed to examine whether a relationship exists between scores on simultaneous and sequential cognitive processes, on one hand, and performance on the Reading Decoding and Arithmetic subtests of the Kaufman Assessment Battery for Children (K–ABC), on the other hand, using a sample of 170 Egyptian school children in Grades 1, 3, and 5. To examine the differential magnitudes of the relationship between scores for cognitive processing and school achievement a two by two (simultaneous × sequential) analysis of variance was calculated with reading decoding and arithmetic scores as dependent variables. The results indicated that cognitive processing, especially simultaneous synthesis, is related to arithmetic as well as decoding during reading. Scores on sequential processing were not significantly related to scores for decoding reading and may not be important as simultaneous processing for mathematical skills. The findings were interpreted in the context of the Arabic orthographic system and in view of the nature of the cognitive and mathematical tasks employed in this study.