The structure of the Si(100)-(1×2) surface, after room-temperature deposition of a thin film of C60, has been examined with grazing-incidence synchrotron x-ray diffraction. The (1×2) surface reconstruction is well preserved under the C60 overlayer. The measured fractional-order diffraction intensities, similar to those reported for the clean Si(100)-(1×2) surface, are analyzed in terms of a dimer model. A similar investigation has been carried out for another (1×2) reconstructed surface, a Cs-doped Ag(110), to test the generality of the preservation of surface reconstructions under a C60 cap layer. In this case, the (1×2) reconstruction is suppressed by the deposition of C60. © 1993 The American Physical Society.