Abstract: A solution phase assay of glycated proteins, such as glycated haemoglobin and plasma-soluble glycated proteins, is carried out by reacting the assay sample with a photoluminescent or chemiluminescent marker compound containing a boronic acid group which reacts selectively with the glycated protein. The luminescence of the marker compound is excited at a wavelength at which it is preferentially quenched by the binding of the marked compound to the glycated protein. The quenching in luminescence gives a measure of the degree of glycation of the glycated proteins.