The development of di- and trihydride phases on gadolinium metallic samples during the initial stage of the hydriding reaction at 100°C and under 0.5 atm of hydrogen was studied using combined microscopic examination, X-ray diffraction (XRD), and hydrogen absorption measurements. Samples were exposed to hydrogen for various times and the amount of hydrogen absorbed was measured. The reaction was then stopped and the samples were removed and examined microscopically to determine the degree of hydride development on the samples surfaces. XRD measurements were performed on the partially hydrided samples. The sequential formation of GdH2 and GdH3 was identified for the first time. The topochemical development of the hydride phases on gadolinium is discussed, based on the experimental results. It was found that following a short nucleation and growth of the dihydride phase, at a relatively early stage of the hydriding reaction, the trihydride is formed on top of the dihydride growing nuclei, so that when the hydride layer formation on the sample is completed, it is already constructed of both hydrides.