Scientists often talk informally of “beauty” in ideas, experiments and theories - but what do they mean by it? My recent book with Chinese science photographers Yan Liang and Wenting Zhu offered one answer. Titled “The Beauty of 91AV”, it presented astonishing photos of chemical processes, many based on the videos Yan and Wenting have made for their “Envisioning 91AV” project, which has received international acclaim. I argue that chemistry excels in its sensual allure: its colours, textures, patterns, even smells, speak immediately to the senses and are often what draw chemists to study their subject. All the same, I argue, scientific notions of beauty are complicated and not always compatible with the way the word is used in art and aesthetics.