'H NMR spectra of ferricytochrome c-551 were obtained in a variety of miscible aqueous buffer-organic solvent mixtures. The pK, of a haem carboxyl group within this protein was measured in several such solvent systems. This work was facilitated by the use of the Super water eliminated Fourier transform (SuperWEFT) technique and this was extended to the suppression of multiple solvent resonances. Nuclear Overhauser enhancement difference experiments were combined with SuperWEFT, enabling structural data to be obtained in complex solvent mixtures. The data arising from this study are relevant to the evaluation and simulation of electrostatic effects in proteins.