The classical Rayleigh model assuming isotopic equilibrium fails to explain the deuterium and oxygen 18 contents of polar snow. This model leads to too high temperature-isotope gradients (both for ΔD and Δ18O), to too low ΔD-Δ18O slopes, and consequently to an excessively large range of deuterium excess and values (d=ΔD-8Δ18O). We present a new model that takes into account the existence of an isotopic kinetic effect at snow formation as a result of the fact that vapor deposition occurs in an environment supersaturated over ice. This kinetic effect is thoroughly discussed from a microphysical point of view and tested against experimental data and field observations. This new formulation reconciles predicted and observed values both for the temperature-isotope and ΔD-ΔD18O relationships for reasonable values of supersaturation over ice.