The dechanneling of 150 keV protons impinging along the <110> direction in nearly perfect copper crystals has been investigated by measurements of the 135° backscattered protons and of the yields of the characteristic Cu(L) X-rays, as a function of the crystal temperature (300 °K, 485 °K, and 645 °K) and the incident beam direction relative to the <110> channel. The data are analyzed and interpreted in a two step procedure. First, the data are reduced in terms of the energies of the observed backscattered protons taking into account the contributions of dechanneled protons and of the protons knocked onto random trajectories by the surface atoms. Second, model backscattering spectra and X-ray yields are constructed by assuming various trial functions for the dependence of the dechanneling rate, on penetration depth. Two trial functions are tested. One is based on a small-angle multiplescattering model of dechanneling. The other assumes an exponential decay of the number of channeled protons, characterized by a mean free path until dechanneling. The data support the latter. The shortening of the mean free path with rising crystal temperature suggests that scattering on the thermally displaced lattice atoms is the primary cause for dechanneling.