Qualitative differences in the behavior of H in Si and Ge have been reported. In Si, shallow acceptors and donors are easily passivated by exposure to atomic H. In Ge, only one unsuccessful attempt at acceptor passivation has been reported and donor passivation was never realized. Further, the amount of D that penetrates into a Si sample exposed to a D plasma increases with the temperature of the plasma. The opposite holds for Ge. Other qualitative differences are apparent from muon spin rotation studies. Our systematic calculations of properties of H in diamond, Si, Ge, and -Sn show that there may be a very simple explanation for all these differences.