This paper, deriving from a larger study, evaluates the readability of two Grade 4 natural sciences textbooks used by learners who speak English as an additional language in two South African schools. The study is set within the context of the reading-related transitional challenges faced by English second language learners when they move from the foundation phase to the intermediate phase. Text readability is critical for educational achievement during this transition. The case study was conducted by means of a qualitative content analysis of factors that are not accommodated in the readability formulae which were used to investigate the textbooks. While the findings from the two instruments were ambivalent for Book 1, with the content analysis showing the book to be largely readable, but the readability calculator indicating it to be beyond the learners' reading level, both instruments indicated poor readability for Book 2. The study recommends a close consideration of text readability by both authors and teachers.