Thermal energy helium and neon diffraction have been used to follow the phase transition of the W(100) surface which reconstructs to a c(2x 2) structure on cooling below room temperature. In the He case we have confirmed previous observations that the reconstructioninduced half-order peaks broaden and shift towards the specular peak on heating through this transition. However, the corresponding Ne diffraction peaks show no appreciable shift or broadening under identical conditions, indicating that thermal He and Ne atoms probe surface structures with different periodicities. Since we would expect that the impinging He and Ne atoms probe quite similar potentials, this result is surprising and the first report for such behavior. These findings are consistent with an anomalous He scattering structure factor or the diffraction of He from a charge-density wave. © 1989, American Vacuum Society. All rights reserved.