Four 1 kg stainless steel weights were used to carry out comparisons to investigate the efficiency of different procedures for cleaning precision mass standards. Not only were mass comparisons carried out on a high-resolution 1 kg comparison balance but also the surface layers of 1 kg mass standards were directly determined for the first time using an automatic ellipsometer. Both measuring techniques show that ultrasonic cleaning in ethanol is the most efficient procedure for cleaning polished stainless steel surfaces. Furthermore, it was possible to prove that ellipsometry as an optical technique of surface analysis is well suited for non-contact, highly sensitive and absolute determination of the surface cleanliness of mass standards.