Resumen
This paper presents a study analyzing second language vocabulary gains after an extensive reading program that included non-fiction graded readers of scientific content in English. The study was conducted in a Spanish primary school (N = 96) and implemented in two different modalities: reading-only and reading-while-listening, which included audiobooks. The study lasted one school year and involved 39 science graded readers, making it unique in its duration and scope. The findings indicate that the practice of extensive reading resulted in notable improvements in vocabulary acquisition during the first half of the school year; however, the advantages were less evident in the second half. Different factors intrinsic to the program but also related to students? motivation will be discussed in order to explain the findings.