The results of an experimental and theoretical investigation of the critical flashing flow of initially subcooled water through rectangular slits are described. The study of such flows is relevant to the prediction of leak rates through cracks in piping or pressure vessels. A simple model has been developed to predict critical mass flux for these flows, which are dominated by the effects of flashing delay (thermal nonequilibrium) and wall friction (due to the large length-to-diameter ratio typically involved). The model is in good agreement with the present data as well as the data of Jeandey et al.