Much has been said and written about the role of extensive reading (ER) in facilitating incidental language learning. However, the relationship between grammar and ER remains noticeably underexplored. As a part of an investigation to identify the potential for incidental grammar learning through ER, this paper uses eye-tracking to investigate grammatical difficulties affecting L2 reading comprehension and the relationship between learners’ L1 background, L2 proficiency and their comprehension of texts. Thirty-five ESL and EFL university students in New Zealand were recruited for the study. Participants were divided into two random groups and guided to read one graded reader and one original text. Evidence from eye movements and stimulated recall interviews helped to reveal grammatical pattern(s) that caused reading comprehension difficulty for ESL/EFL learners. This study would have implications for curriculum designers, language teachers and extend the understanding of the relationship between learners’ L1 background and their comprehension of L2 texts.