The compound Ag₂O undergoes large and isotropic negative thermal expansion over 0–500K. We report temperature dependent in elastic neutron scattering measurements and ab-initio calculations of the phonon spectrum. The temperature dependence of the experimental phonon spectrum shows strong an harmonic nature of phonon modes of energy around 2.4meV. The ab-initio calculations reveal that the maximum negative Gr¨uneisen parameter, which is a measure of the relevant anharmonicity,  occurs for the transverse phonon modes that involve bending motions of the Ag₄O tetrahedra. The thermal expansion is evaluated from the ab-initio calculation of the pressure dependence of the phonon modes, and found in good agreement with available experimental data.