Protactinium trihydride is an inorganic compound with the chemical formula PaH3. It is isostructural with uranium trihydride and can be prepared by reacting protactinium and hydrogen at 250°C and 600 mmHg.[1] Theoretical calculations show that it can form further compounds PaHn (n = 4, 5, 8, 9) under high pressure.[2] Protactinium trihydride is sensitive to moist air and oxygen.