It is well recognized that pH plays a significant regulatory role in most cellular processes. Increasingly, there is interest in transmembrane pH gradients, particularly with respect to tumor growth and response to therapy. NMR offers a non-invasive approach to monitoring cellular pH and detecting changes in response to interventions. This review will consider the strengths of various approaches to measuring pH with particular focus on the reporter molecules designed to interrogate the cellular milieu. In particular, fluorinated vitamin B6 derivatives (6-fluoropyridoxol and 6-fluoropyridoxamine) will be described, which for the first time provide a practical non-destructive method to measure simultaneously intra- and extracellular pH, i.e., the transmembrane pH gradient in animals in vivo based on a single reporter molecule.