Pressure-composition isotherms of PdH x films of 100-400 nm thickness have been measured between 300 and 500K at pressures up to 10 bar by means of an oscillating quartz microbalance. For all the films investigated the two-phase region is considerably narrower than that of bulk PdH x. This narrowing of the miscibility gap, together with the observed lowering of the critical temperature, is due to a weakening of the hydrogen-hydrogen interaction which probably originates from the elastic boundary conditions at the film-substrate interface.