The ignition delay and combustion characteristics of hydrogen jets in an argon-oxygen atmosphere were investigated to provide fundamental data for operating an argon-circulated hydrogen internal combustion engine. Experiments were conducted in a constant-volume combustion vessel to study the effects of ambient temperature, ambient pressure, oxygen concentration and injection pressure on a pre-burning system. The hydrogen-jet penetration and flame were also investigated based on high-speed shadowgraph images. The experimental results indicated that the ignition delay (τ) increases as the ambient temperature (T_i) decreases, similar to the results obtained in an air atmosphere. The heat-release rate results also exhibited similar trends. In addition, other fundamental characteristics were investigated by modifying the oxygen concentration (r_O2), ambient pressure (p_i) and injection pressure (p_j) conditions, which might help in controlling the operation in an argon-circulated hydrogen engine.