AbstractIn this work, we studied a few surface super‐structures using reflection high‐energy positron diffraction (RHEPD). We found that the RHEPD pattern from the Si(111)‐7×7 surface exhibits different features from that of electron diffraction. The difference is explained as a strong thermal diffuse scattering of positrons at the first surface layer. We also studied the phase transition of the Si(111)‐√3×√3‐Ag surface. We found that the surface sensitive diffraction spots exhibit peculiar temperature dependences. Based on theoretical considerations, we interpreted the temperature dependences in terms of the order‐disorder phase transition. Furthermore, we succeeded in determining a unique structure of the Si(111)‐√21×√21‐Ag surface. (© 2007 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)