Previous work has shown that a surface wave on amorphous o-terphenyl (OTP) decays by viscous flow at high temperatures and by surface diffusion at low temperatures. We report that the surface mass transport can be efficiently suppressed by low-concentration polymers. Surface-grating decay has been measured for OTP containing 1 wt % polystyrene (PS, M-w = 1-8 kg/mol), which is miscible with OTP. The additive has no significant effect on the decay kinetics in the viscous-flow regime, but a significant effect in the surface-diffusion regime. In the latter case, surface evolution slows down and becomes nonexponential (decelerating over time). The effect increases with falling temperature and the molecular weight of PS. These results are attributed to the very different mobility of PS (slow) and OTP (fast) and their segregation during surface evolution, and relevant for understanding the surface mobility of multicomponent amorphous materials.