A process is provided for determining the functional residual capacity (FRC) of the lungs during respiration. An environmentally friendly trace gas is used in a process and system for determining the FRC by using fluoropropanes as a trace gas. Values for the FRC can thus be calculated, resolved for individual breaths, from the expiratory trace gas concentration and the expired breathing gas volume and they can be used for determining the FRC depending on their convergence behavior.