The high-temperature oxidation behaviour of chromium-molybdenum-vanadium alloyed hot-work tool steel was investigated. High-temperature oxidation was investigated in two conditions: soft annealed, and quenched and tempered. The samples were oxidised in a chamber furnace and in an instrument for simultaneous thermal analysis, for 100 h in the temperature range between 400 °C and 700 °C. Metallographic analysis (optical and scanning electron microscopy) was performed to study the microstructural changes in the steel and the oxide layer. Oxidation kinetics were analysed by thermogravimetric analysis, and equations were derived from the results. The kinetics can be described by three mathematical functions, namely: exponential, parabolic and cubic. However, which function best describes the kinetics depends on the oxidation temperature and the thermal condition of the steel. Quenched and tempered samples were shown to oxidise less, resulting in a slower oxidation rate.