When readers are asked to detect a target letter while reading for comprehension, they miss it more frequently when it is embedded in a frequent function word than in a less frequent content word. This missing-letter effect has been used to investigate the cognitive processes involved in reading. A similar effect, called the missing-phoneme effect has been found in aural language when participants listen to the narration of a text while searching for a target phoneme. In three experiments, we tested the hypothesis that both effects derived from the same cognitive processes, by isolating the role of word frequency and word function. In Experiment 1, we used a paper and pencil procedure for reading and a continuous narration for the listening task. In Experiments 2 and 3, we used a rapid serial visual or auditory presentation procedure to control for the effects of preprocessing upcoming information: parafoveal processing in reading and coarticulation in aural language processing. Parallel findings were observed in the reading and listening tasks. In all experiments, there was an effect of word function, and there was an effect of word frequency in Experiments 1 and 3. Results are interpreted in light of the attentional disengagement model.