This research investigates the relative importance of vocabulary and oral reading fluency as measurement dimensions of reading comprehension as the student passes from elementary to high school. Invariance of this model over grades 4 through 8 is tested using two independent student samples reading grade-level appropriate passages. Results from structural equation modeling indicate that the model is not invariant across grade levels. Vocabulary knowledge is a significant and constant predictor of overall reading comprehension irrespective of grade level. While significant, fluency effects diminish over grades, especially in the later grades. Lack of grade level invariance was obtained with both samples. Results are discussed in light of vertically linked reading assessments, adequate yearly progress, and instruction.