Formant dynamics are an interesting source of speaker-discriminating information, reflecting both differences in speakers' vocal tract morphology and individual differences in the articulatory trajectories chosen to produce each sound. A study of speaker-distinguishing properties of the formant dynamics of /u:/ in Standard Southern British English is presented. Measurements at equidistant intervals along the F1 and F2 contours are compared with polynomial characterisations of the contours. Approximating the contours with quadratic and cubic polynomials allows more speaker-discriminating information to be conveyed with fewer parameters. The best value per predictor is provided by the cubic approximation of F2.