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The positive ion current emitted by heated filaments of high‐purity Fe displayed sharp maxima when the metal passed through the structural transformation temperature. The height of the maxima varied with previous flashing temperatures and with the direction of approach to the transformation. Plastic deformation also produced maxima in the positive ion current after a temperature‐dependent time delay; the peak height was approximately proportional to strain. Electron emission from the same filaments was unaffected by these factors. The results suggest that the positive ion emission from flash‐cleaned Fe filaments is not surface controlled and they seem to require internal rate controlling mechanisms which involve diffusion of the impurity species to the surface along high diffusivity paths.