A new mass conservation equation for boiling flow, which takes properly into account the slip between the phases, is proposed. The fact that, due to continuity, both phase velocities can be used to derive the new mass balance leads to the constancy of the slip ratio. Also new, coherent mass quality-void fraction relationships are derived. Correspondingly, a new energy balance for boiling flow is stated. The new expressions are successfully compared against void fraction measurements in bulk and subcooled boiling flow.