The purpose of this study is to equate and further validate three forms of the vocabulary size test (VST) created by Aizawa and Mochizuki (2010). These three forms, VST 1, 2, and 3, were administered to a cohort of 189 high school students ranging in age from 16 to 18 in April of their 1st, 2nd, and 3rd year of high school. Although these alternate forms were designed to be of equal difficulty, formal equating of the three forms was never carried out. In order to verify whether gains in test scores were due to growth in vocabulary size or differences in difficulty among the three forms, a fourth form comprised of items selected from VST 1-3 was created and administered in December of year 3. The four test forms were then equated using Rasch analysis, placing persons and items on a single, uniform logit scale. The results indicated that (1) the three original forms of the VST all showed good fit to the Rasch model, (2) differences in test difficulty among the original three forms were minor, and (3) the four VST forms, linked via a single Rasch analysis, can be appropriately used for estimating gains in students' VS across their high school career. In addition, follow-up analyses indicated considerable overlap in item difficulty among word frequency bands, suggesting that word frequency was not the sole indicator of difficulty of vocabulary items, and that learner progress in vocabulary learning tended to be uniform and parallel across all frequency bands. Overall, the study illustrates a method for creating a valid and reliable measure of growth in VS over an extended period of time and provides insight into the relationships among word frequency, word difficulty, and progress in vocabulary learning.