Two experiments assessed the effect of vocabulary difficulty on three measures of text comprehension���free recall, summary recall, and sentence recognition. In Experiment 1, the effects of differing proportions of rare-word substitutions were examined. It was found that a high rate of difficult vocabulary (one substance word in three) was required before reliable effects on comprehension were evident. In Experiment 2, difficult vocabulary was placed in important text elements in one form of the passages, and in unimportant elements in another. These forms were contrasted with easy vocabulary forms in their effects on the three comprehension measures. Only on the summary measure was there an overall effect of difficult vocabulary in important elements. The results are discussed in terms of the salience of the signaling value of unfamiliar words. �� 1983, SAGE Publications. All rights reserved.