We have measured dopant distribution profiles by spreading resistance and secondary ion mass spectroscopy (SIMS) profiling at silicon surfaces annealed to high temperature in vacuum. Accumulation of electrically active p-type dopants is found to occur. The p-type dopant is identified by SIMS and by its diffusion characteristics as being boron. The measured doping profiles can be interpreted as indicating that the chemically cleaned surface contains boron impurities in the 1012/cm2 range that become electrically activated and diffuse into the substrate during high-temperature anneals. It is concluded that the boron exists initially in an oxidized state at the surface. Borosilicate parts present in any standard ultrahigh vacuum system are identified as a source of the pollution.