Separation measurements have been made for neon isotopes penetrating a nitrogen jet over a broad range of flow regimes. The products of the jet‐background interaction were collected by molecular probes which projected through the jet orifice from the plenum into the background chamber. The separation profiles are in excellent agreement with a very simple physical model of the penetration process. This model can be used to predict the performance of any isotopic separation experiment and has been shown to agree with another investigator’s argon and SF6data. The amount of separation is adjustable, does not appear to be limited, and thus can be arbitrarily large. The apparatus required is particularly simple to construct.