Evolution of pulses obtained by modulating spatial optical solitons in a Kerr no nlinearity planar waveguide with anomalous dispersion is investigated. The pulse s will collapse, self_trapping or divergence according as its initial temporal w idth is more than, equal to or less than the critical value. The higher the orde r of the spatial soliton pulses, the smaller the critical value. The pulse colla pses fastest when its initial temporal width is equal to a special value, and th e more the difference between its initial temporal width and the special value, the slower it collapses. The higher the order of the spatial soliton pulses, th e smaller the special value. The peak power density, temporal and spatial width of the self_trapped pulses differ distinctly from their initial values