This paper reports activities (as part of a university course in language teacher education on teaching reading) in which primary school student teachers (all ESL in-service teacher trainees) explored their own skills of determining textbook readability using an online software tool and a cloze test completed by two hundred and seventy-eight Grade Seven primary school pupils. Findings from the online tool were that the text was difficult. The cloze test confirmed this when it showed that only eighteen pupils could read the text unassisted while the rest were frustrated by it. The paper uses these findings to describe the challenges pupils face and how readability research is beneficial to the reading development of ESL learners if reading of academic texts is approached from the principles of the interactive view of reading; of cognitive learning theory; and of second language acquisition theory. It is concluded that teachers' awareness of readability issues is helpful for effective reading instruction during the critical formative years of school.