We demonstrate a technique which can change the pH inside an NMR tube in steps of <= 0.01 pH unit, during a solution-state NMR experiment. We use a photo-acid co-solute with the actual specimen, and photo-excite the solution by direct fibre-mediated light delivery to the NMR tube. The photo-excitation and NMR measurements can be performed simultaneously. Our method should allow precise variation of the pH in studying protein folding, aggregation and enzyme catalysis processes which often display critical pH dependence. (C) 2018 Elsevier B.V. All rights reserved.