Infrared reflectivity was measuredinsitufrom single phase layers of Rb1C60with a thickness of about 4 &mgr;m on a {111} face and of about 0.5 &mgr;m on a {100} face of C60single crystals. The fcc→orthorhombic phase transition was traced by a splitting of theF1u(4) mode into three lines within a temperature range of 2° at 404/415 K for a cooling/heating cycle, respectively. The reflectivity spectra remained unchanged after exposing the orthorhombic Rb1C60to air. Even the fcc phase appears to be vibrationally stable in air up to at least 450 K. The conductivity of the orthorhombic phase shows a weakly activated temperature dependence. ©1995 American Institute of Physics.