A rapid, easy to handle and low cost assay of ascorbic acid was developed, which consists in miniaturization on microplate of a redox reaction monitored spectrophotometrically, the signal being directly related to the concentration of ascorbic acid in the sample. This method was investigated in accordance with the prerogative of regulation covering food additives in terms of linear range/linearity, limits of detection and quantification, accuracy (including trueness and precision), robustness and selectivity. The application of the method was tested to determine ascorbic acid diffusivity in agar gel used as an aqueous food simulant. The diffusivity value obtained was 0.31 x 10[puissance 9] m² s[puissance -1], with a confidence interval of 20% at 95% level. Accuracy of the assay and of the point's location on ascorbic acid profiles were examined by Monte-Carlo method to assess their influence on accuracy of diffusivity value obtained. Assay uncertainty was clearly demonstrated to be a critical parameter.