Surface phenomena which occur in the process of thin yttrium hydride film YHx (0 < x < 2.62-2.80) formation were studied through simultaneous measurements of work function Phi and hydrogen pressure P during H-2 interaction "in situ" with thin yttrium films, deposited under ultrahigh vacuum conditions. These studies were correlated with the results of parallel measurements of thin yttrium film electrical resistance and hydrogen pressure and optical observations. The experiments were performed over the pressure and temperature intervals: 10(-8)-10 Pa and 78-298 K, respectively. Thin YHx films (0 < x < 2.62-2.80) were easily obtained within the temperature interval 176-298 K, while at 78 K the atomic ratio x = H/Y did not exceed 0.14. At low temperatures, consumption of hydrogen occurred without any measurable work function changes. This suggests generation of a symmetrical H-Y-H adspecies on the surface, reassembling the alpha' phase known for the bulk of YHx. This adsorbate is stable at low temperature and inhibits the incorporation of hydrogen into the bulk. The features of Phi (t) and P(t) changes observed above 176 K suggest the formation of YHx via several successive surface processes: (i) generation of the H-Y-H adsorbate of a low degree of short range order; (ii) creation of positively charged beta (+) hydrogen adspecies which incorporates quickly below the surface; (iii) arising of YH3 precipitates on the surface, accompanied by an increase of work function.