We have used composition depth profiling, based on nuclear-reaction analysis and secondary ion mass spectroscopy, to study segregation at the free surface of a partly miscible binary mixture consisting of random olefinic copolymers. The equilibrium surface excess data, analysed within a mean-field Cahn approach, point to a wetting transition. The surface phase diagram obtained was confirmed by the observed dynamics of the segregation from a coexistence composition: the monotonic and halted growth of the surface layer was observed at temperatures above and below the predicted wetting point, respectively.