In a method for determining a low pressure exhaust gas recirculation mass flow {dot over (m)}Opt in the air system of an internal combustion engine (10) with low pressure exhaust gas recirculation, the determination of the low pressure exhaust gas recirculation mass flow {dot over (m)}Opt involves a preliminary low pressure exhaust gas recirculation mass flow {dot over (m)}EGRLPDs which is determined from a function involving, in particular, the pressure p42 upstream and the pressure p12 downstream of a low pressure side exhaust gas recirculation valve EGRVIvLP (29), the temperature TEGRVIvPUs downstream of the low pressure side exhaust gas recirculation valve (29), and the geometry arEGRVIvLP of the low pressure side exhaust gas recirculation valve (29). According to the invention, at least one mass balance based variable is also considered in the determination of the low pressure exhaust gas recirculation mass flow {dot over (m)}Opt.