Measurements of effective diffusivity by the chromatographic method are shown to be biased by nonuniformity of the external porosity in a cross-section of the column. A very simple model is proposed to interpret experimental results. A modified Van Deemter method is given to extract the axial dispersion and effective diffusion coefficients from chromatographic peaks. This method allows one to ignore the external porosity distribution. Experiments show that even though axial dispersion obeys classical correlations, the internal mass transfer parameters may be strongly affected by the external porosity distribution.