The lag-time between sample light exposure and evaluation of the samples strongly influences the observed change in surface color of riboflavin bulk substance and tablets. The decrease in total color change (deltaE*ab) as a function of time after exposure is ascribed to a change in the surface moisture content of the samples during storage. Our results emphasize the need for a standardization of the lag-time between light exposure and sample evaluation in the photostability testing protocol of drug substances and products.