Gravitational acceleration has been measured with high precision using macroscopic probes and for a few quantum probes. A recent diffraction experiment (Arndt M et al 1999 Nature 401 680) has demonstrated the wave-particle duality for the C₆₀ fullerene molecules. In this paper, a new method to measure gravitational acceleration with fullerene molecules is proposed. The g value is deduced from the measurement of the phase shift of a thermal beam in a three-grating Mach–Zehnder interferometer. The experimental setup and the uncertainty sources are discussed. A measurement with a precision of 1 × 10⁻⁴ is feasible with 10⁸ thermal fullerene molecules and gratings with 500 nm period. The precision achievable by the proposed method is limited by the velocity spread of the fullerene beam.