A study investigated how text features influence the amount of vocabulary knowledge students acquired incidentally while reading expositions. Three sets of text features were identified from studies on comprehension: (1) features associated with the macrostructure, (2) features associated with logical and temporal relations in the microstructure, and (3) features associated with explanations of concepts and relations among them. Two natural expositions were revised in three successive, incremental steps, yielding four versions for each exposition. The most difficult words in each exposition were identified. Approximately 300 eighth grade students read one text version and completed a multiple choice test written to be sensitive to small gains in word knowledge. The results indicated that both able and less able readers who read text versions in which key concepts and the relations among them had been explained thoroughly learned significantly more word meanings than did students who read any of the other versions. (Author/FL)