Abstract: During a process involving one or more process steps of a process phase, in which a substrate is located in the process chamber of a CVD reactor, a process temperature and a pressure are each set and a process gas flow is fed into the process chamber by way of control data delivered by a controller in accordance with a formula stored in the controller. Additionally, sensors are used to determine measurement data from which a current fingerprint is calculated and then compared with a historic fingerprint. The fingerprint includes only values or groups of values that are obtained from measured values that are recorded during one or more conditioning steps of a conditioning phase in which a conditioning temperature and a conditioning pressure are each set and a conditioning gas flow is fed into the process chamber in accordance with control data specified by the formula.