Two important aspects of short-term thermally stratified energy storage, thermocline mixing and thermocline thickness, are studied analytically, experimentally and numerically The storage detrimental aspects are investigated for a simplified configuration, i.e. an adiabatic box containing a quasi-stationary thermocline. Numerical finite difference/volume simulations agree well with experiments. The dissipation-free 1D analytical model shows a large discrepancy. It appears that mixing inside thermally stratified stores is a two-stage process. First fluid is withdrawn from the thermocline by viscous drag. Subsequent mixing takes place by stretching and folding of fluid particles, thereby enabling diffusion to become active.