The reactions of Mn atoms with water and MnO with H-2 have been studied using matrix-isolation FTIR and DFT theoretical calculations. In solid argon, Mn atom reacted with H2O molecule to form the MnOH2 complex spontaneously. Photolysis destroyed the MnOH2 complex and produced the inserted HMnOH molecule. In addition, HMnOMnH, Mn(OH)(2), HMnOMnOH, and posssibly HMnOMnOMnH molecules were also produced on photolysis. Reaction of MnO with H2 gave the (H2)MnO complex without activation energy. Photolysis of the (H2)MnO produced the inserted HMnOH molecule as well. The aforementioned species were identified via isotopic substitutions as well as density functional theoretical frequency calculations. Potential energy surface for the Mn + H2O <----> MnO = H-2 reaction was also given. Our calculation results showed that all these reaction intermediates and products are high spin molecules. The MnOH2, HMnOH and (H-2)MnO molecules have ground states with S = 5/2, the HMnOMnH, and HMnOMnOH molecules have ground states with S = 10/2, and the HMnOMnOMnH molecule has ground state with S = 15/2.