The present study affords an explanation for the consistent, but not always statistically significant, pattern showing superior verbal discrimination learning performance for low-as compared to high-frequency words. In a frequency judgment task, it was found that relative to high-frequency words, low-frequency words for which Ss (sixth graders) knew the meanings produced apparent frequency measures consistent with superior verbal discrimination learning, while low-frequency words that were unknown to the children did not. These results, taken together with those based on comparisons of pictures and high-frequency words, lend themselves to a modified Weber's law interpretation of stimulus material differences in discrimination learning.