Abstract: CO2G0 is a novel method to automatically estimate in real-time a person's CO2 emissions associated with transportation mode choices using data—specific inertial information gathered from mobile phone sensors. CO2G0 automatically classifies the user's transportation mode among eight classes by using a Functional Tree. The algorithm is trained on features gathered from an accelerometer, GPS receiver and digital maps. A working smartphone application for the Android platform has been developed and experimental data have been used to train and validate the proposed method. A second algorithm computes the traveled distance, through an optimized mix of GPS and Internet map services.