The authors investigated how text features may influence the amount of vocabulary knowledge acquired incidentally while reading expositions. Three sets of text features were identified from studies on comprehension: (a) features associated with the macrostructure, (b) features associated with logical and temporal relations in the microstructure, and (c) features associated with explanations of concepts and relations between 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. Eighth-grade students (N = 309) read one text version and completed a multiple-choice test written to be sensitive to small gains in word knowledge. Both the able and the less able students who read versions in which key concepts and the relations between them had been explained thoroughly learned significantly more word meanings than students who read any of the other versions.