Neutron diffraction and quasielastic neutron experiments are performed to investigate the effect of HCl adsorption on the structure and dynamics of an ultra-thin ice Ih film (5 H2O bilayers thick) deposited on a crystalline MgO(001) substrate. Three HCl coverages have been studied 0.3, 0.6, and 1 monolayer (ML) in the temperature range 190-270 K. At 0.3 and 0.6 HCl monolayer, no mobility is measured at Tless than or equal to220 K. A translational mobility, which is a signature of a liquid phase, is observed at T=250 K. This phase occurs 15 K below the surface melting temperature of the bare ice film. The fraction of mobile molecules represents 30% (0.3 ML HCl) and 45% (0.6 ML HCl) of the film. At 1 HCl monolayer and T=220 K, HCl-dihydrate coexists with ice Ih, whereas at T=250 K the ice film becomes amorphous and only 9% of the film is mobile. The results are discussed within the context of atmospheric chemistry.