We report two experiments investigating the masked onset priming effect (MOPE) in reading aloud. More specifically, we tried to provide an answer to the question of whether or not mismatching segments in the prime have an inhibitory effect on the MOPE. Dutch native speakers saw four-letter target words preceded by visually masked primes that either consisted of whole words or letters, and either matched or did not match the onset segment of the target. Prime exposure duration was varied between 33 ms and 66 ms to investigate the time course of the obtained effects. Whole-word primes behaved the same as letter primes at the short (33 ms) prime exposure duration, whereas at longer prime exposure (66 ms) effects of mismatching segments present in the whole-word but not in the letter primes led to slower overall naming latencies, suggesting that inhibition from segments beyond the onset needs time to build up.