Experimental study of a Nanosecond Repetitively Pulsed (NRP) discharge in atmospheric pressure water vapor at 450K is reported. The discharge is produced between two pin electrodes by application of voltage pulses (0-15 kV amplitude), 10 ns in duration, with a repetition frequency up to 30 kHz. Electrical measurements were done to determine the energy deposited in the discharge. The energy per pulse ranges from 1 to 10 mJ. In order to determine the efficiency of water vapor dissociation, the concentration of the final reaction products (H2 and O2) and their respective flow rates were measured.