A method and a system for ascertaining a mass value representing the vehicle mass of a motor vehicle, particularly of a commercial vehicle, having a drive unit. An acquisition at least of a first and a second acceleration value is provided for this determination. These acceleration values represent the vehicle acceleration at a first and a second point of time. Further, at least one first and one second drive value are acquired. These drive values represent the drive force or the drive torque of the drive unit at the first and the second point of time. At least as a function of the acquired acceleration values and the acquired drive values, at least a first and a second driving-resistance value are then determined. The mass value is ascertained at least as a function of a comparison of the determined first driving-resistance or mass estimated value to the determined second driving-resistance or mass estimated value. A roadway slope is detected using the comparison, whereby an erroneous mass determination caused by the roadway slope is avoided.