In order to calculate the limiting magnetic field for superconductivity of a type II superconductor, the free‐energy contributions of the Abrikosov mixed state and the electron spin paramagnetism are combined. The solutions which result are applied to the niobium—zirconium alloy system and compared with the available resistive critical field data, as a function of composition. The calculated curve is generally closer to the experimental curve than either the paramagnetic limit orHc2alone, and rather good agreement is obtained for niobium‐rich alloys.