Traditional reading comprehension tests have shown sizable Black-White mean subgroup differences. In this paper, I argue that part of the reason for this phenomenon lies in the atheoretical nature of existing tests and that the SIENA Reading Component Process Test© (RCPT), a new, theory-driven measure the cognitive components of reading comprehension shows reduced subgroup differences while still exhibiting a substantial relationship with a traditional reading comprehension test. Furthermore, I hypothesize that subcomponents of the SIENA RCPT© that rely on prior knowledge show greater subgroup differences than those subcomponents that do not require access to prior knowledge. Consistent with my hypothesis, the new SIENA RCPT© overall shows reduced subgroup differences compared to a traditional reading comprehension measure and evidence for convergent validity for the SIENA RCPT© is also found. Contrary to my hypothesis, the subcomponents of the SIENA RCPT© that rely on prior knowledge show less subgroup differences than those subcomponents that do not require access to prior knowledge.