Study of Band Broadening occurring in Size Exclusion Chromatography (SEC) is reported using very narrow PS standards obtained and characterised by Temperature Gradient Interaction Chromatography (TGIC). Chromatograms are fitted by Exponentially Modified Gaussian functions (EMG) and mapping of band broadening is obtained for different column sets. Interpretation of the skewing of the chromatograms is proposed with a new model using Brownian motion properties inside the pores. That explains why band broadening and tailing become so important near total exclusion volume.