Internal combustion steam cycle (ICSC) is a novel steam power cycle using hydrogen as an energy carrier to produce superheated steam. High humidity hydrogen produced during fast hydrogen production process is directly used to produce superheated steam by combusting with stoichiometric oxygen without hydrogen storage. The ICSC efficiency is greatly affected by the content of non-condensable gas in superheated steam. In the present study, superheated steam generation by high humidity hydrogen was investigated in a model internal combustion steam generator. Effects of H2O/H-2 molar ratio of humid hydrogen and velocity ratio of humid hydrogen to oxygen on non-condensable gas content, combustion efficiency, and mixing rate were evaluated. The results showed that the critical H2O/H-2 ratio for the humid hydrogen humidity limit was 2.8. With increasing velocity ratio, mixing rate and combustion efficiency increased under the same H2O/H-2 ratio. The H2O/H-2 reaction rate monotonously decreased as the H2O/H-2 ratio increased from 1.0 to 2.5, while the mixing rate increased along with the velocity ratio. The combustion efficiency initially increased and subsequently decreased, and the peak value was reached at a H2O/H-2 ratio of 1.75. This result indicated that the humid H-2-O-2 combustion was controlled by diffusion under H2O/H-2 ratios of 1.0 to 1.75, but turned to be controlled by chemical kinetics when the H2O/H-2 ratio ranged between 1.75 and 2.5.