Fragmentation is one of the mechanisms involved in rock avalanches. Quantifying the associated energy during a rock avalanche can help to assess the influence of fragmentation on post-failure mass movements. In this paper, in situ block size distributions of the intact rock mass and the debris deposits are presented and analyzed for nine rock avalanches, five in the Canadian Rocky Mountains and four in the European Alps. Degrees of fragmentation are estimated from these data. Two methods are examined to assess fragmentation energy, one based on the comminution theory, and one on the blasting energy used in the mining industry. The results show that, for the studied rock avalanches, there is a relationship between the reduction in diameter ratio, Rr = D50/d50 (where D50 and d50 are the mean diameter of the intact rock mass and the mean diameter of the debris, respectively), and the potential energy per unit volume normalized with respect to the point load strength of rock (γHG/σc), where γ is the unit weight of the material, HG is the vertical distance between the centres of gravity of the mass at the start and end positions, and σc is the point load strength). For the cases studied, fragmentation energy calculations average 20% of the potential energy. An empirical relationship between Rr and γHG/σc has been established and is used in the definition of a disintegration index (ID). This index seems to reflect the physics of the disintegration process, since it accounts for the fact that the reduction in diameter ratio is a function of the dissipated energy and the strength of the rock. These factors have long been known to affect fragmentation but have never been presented in a coherent manner for rock avalanches.Key words: rock avalanches, disintegration, fragmentation energy, Canadian Rocky Mountains, European Alps.