Previous research indicates that pairing pictures with associated words when teaching sight-word reading may hinder acquisition (e.g., Didden, Prinsen, & Sigafoos, 2000; Singh & Solman, 1990; Solman & Singh, 1992). The purpose of the current study was to determine whether this phenomenon was due to a previously learned association between the spoken word and picture (i.e., blocking) or due to the mere presence of a picture as an extrastimulus prompt (i.e., overshadowing). Three participants were taught to recognize words that were presented alone or paired with pictures that the participants either could or could not identify prior to training. All participants learned the words more quickly when they were presented alone rather than with pictures, regardless of their prior learning history with respect to pictures representing the words. This finding is consistent with the phenomenon of overshadowing. Nonetheless, consistent with blocking, all participants also acquired the words presented alone more quickly if they could not identify the associated pictures prior to training. Together, these findings have important implications for using prompts when teaching skills to individuals with developmental disabilities. (Contains 2 figures and 1 table.)