The boron isotope pH proxy is increasingly being used to reconstruct ocean pH in the past. Here we detail a novel analytical methodology for measuring the boron isotopic composition (δ¹¹B) of diatom opal and apply this to the study of the diatom Thalassiosira weissflogii grown in culture over a range of pH. To our knowledge this is the first study of its kind and provides unique insights into the way in which diatoms incorporate boron and their potential as archives of palaeoclimate records.