This work was performed to study the correlation between the Surface and bulk phenomena that occur during H-2 interaction with terbium, leading to three-hydride TbHx (x approximate to 3) formation. This reaction is accompanied by the transition of the original metal into a semiconductor. It was found that thin films are particularly useful for such studies. Measurements of work function changes Delta Phi(H/Tb) were chosen to illustrate the Surface phenomena, and the relative electrical resistance R(H/Tb)/R-0 and light transparency T(H/Tb)/T-0 correspond to the bulk properties. Additionally, BET experiments were performed to determine the influence of three-hydride formation oil the area of a thin Tb film. It was observed at 298 K that a precursor state of the adsorbate arose at the beginning of the reaction, when (H/Tb < 0.1), decreasing the work Function by Delta Phi = 12 mV. A higher Uptake of hydrogen caused all increased work function, followed by Delta Phi transients. This has been interpreted as local hydride formation on the surface and its expansion into the bulk, until a concentration of H/Tb approximate to 3 was reached. TbHx (x similar to 3) formation resulted in Delta Phi = -200 mV and an increase in the thin film area by a factor of similar to 3. These phenomena were accompanied by characteristic changes in the bulk properties. The light-reflecting thin Tb film was transformed into a transparent hydride, with an similar to 23-fold increase in R/R-0. At 78 K, only a small amount of hydrogen (H/Tb = 0.13) was consumed, leading to Delta Phi = -23 mV. This uptake is stable up to 100 K. Increasing the temperature above this Value resulted in the additional large absorption of hydrogen. This could suggest the formation of a low-temperature surface phase of the hydride.