The stability of ZrNi and ZrCo to heat cycles in hydrogen atmosphere was studied through changes in absorption-desorption characteristics and in crystallo-graphic structures. ZrCo easily lost its absorption- desorption capacity of hydrogen below 30 heat cycles between room temperature and a given temperature in a range of 400 ∼600 °C. X-ray diffraction analysis showed that ZrCoH3 initially formed decomposed to ZrH2+ ZrCo2. On the other hand, ZrNi was more durable than ZrCo to the similar heat cycles. But, it was found that the absorption-desorption characteristics was degraded by heat cycles over 500. The X-ray analysis showed that ZrNi also dispropor-tionated to ZrH2 and ZrNi3. The difference in the stabilities between the two materials appears to be due to the difference in crystallographic nature upon formation of the respective hydrides.