•Temporal evolution of Fuchs-Kliewer phonon spectra of pristine GaN(1-100) surfaces.•Monotonous decrease of multi-phonon scattering probabilities is not applicable.•Dissociative water adsorption is likely to change surface dielectric constants. Display omitted Inelastic electron scattering is used to study surface phonon polaritons on H-covered and clean GaN(11¯00) surfaces. The Fuchs–Kliewer phonon of GaN(11¯00)–H gives rise to characteristic signatures of its single and multiple excitation in specular electron energy loss spectra. The loss intensities for multi-phonon scattering processes decrease according to a Poisson distribution. Vibrational spectra of this surface are invariant on the time scale of days reflecting its chemical passivation by the H layer. In contrast, vibrational spectra of pristine GaN(11¯00) are subject to a pronounced temporal evolution where spectroscopic weight is gradually shifted towards the multiple excitation of the Fuchs–Kliewer phonon. As a consequence, the monotonous decrease of the cross section for multiple quantum excitation as observed for the H-covered surface is not applicable. This remarkable effect is particularly strong in spectra acquired at low primary energies of incident electrons, which hints at processes occurring in the very surface region. Scenarios that may contribute to these observations are discussed.