Time-of-flight mass spectrometry is used to measure the translational kinetic energy release during the unimolecular decay of Na-n(+) (n = 5-21) clusters. The detection of neutral fragments of evaporation allows measurement of translational kinetic energies as small as 10 meV. The kinetic energy release predicted in the frame of a harmonic model is compared with the measured value. The discrepancy, interpreted in the frame of a novel extension of phase-space theory, emphasizes the effect of the sub-cluster rotation in the kinetic energy release and the role of vibrational anharmonicity.