Quantitative assessment of a disproportionation in the ZrCo-hydrogen system under ITER-relevant operating conditions was performed by means of experimental tests and a theoretical calculation. In the static temperature experiments with equilibrium hydrogen pressures, a 10% disproportionation of ZrCoHx (x = 2.0 and 2.5) was observed in 5.5 h at 415°C (~78 kPa), 9 h at 400°C (~72 kPa), 172 h at 380°C (~51 kPa), and 1626 h at 350°C (~28 kPa). An experimental formula [log = 17 268/T (K) - 25.814, where is the reaction time (day) of 10% disproportionation] was derived from these experiments. Experiments with a temperature cycling of up to 125 cycles (from room temperature to 350 to 360°C) proved that no enhancement of a disproportionation occurs in the ZrCoHx (1.7 < x 2.0). Typical operation conditions of the ZrCo hydride bed for the D-T gas storage delivery system were proposed based on the ITER FDR 2000 plasma operation scenarios. The disproportionation rate estimated conservatively by the theoretical model indicates that a disproportionation in the ITER basic performance phase can be reduced by <4% even when there is a direct supply from the fuel storage and delivery system beds for all the D-T pulses and by <0.1% when the supply is from the hydrogen isotope separation system.