Several radioactive isotopes of Fe and Co were simultaneously diffused into an equiatomic FeCo alloy which exhibits fcc, bcc, and CsCl-type ordered phases with decreasing temperature. Penetration profiles were obtained by standard lathe sectioning and a grinding-sectioning technique described in this paper. From In D-versus-1T plots, activation energies of 68.5 and 55 kcal and frequency factors of 1.26 and 0.25 cm2/sec were obtained for diffusion of Fe59 in the fcc and bcc phases, respectively. From similar plots, activation energies of 69.4 and 60.0 kcal and frequency factors of 1.33 and 2.00 cm2/sec were obtained for diffusion of Co57 in the fcc and bcc phases of the FeCo alloy. The average isotope-effect parameters were 0.66 in the fcc phase and 0.52 in the bcc phase. Manning's theory for concentrated alloys was used to calculate correlation factors for the FeCo alloy. From these calculated values and experimentally measured isotope-effect parameters, K was determined to be slightly larger in the fcc and bcc phases of the FeCo alloy than has been reported in pure Fe. The observed decrease in the isotope-effect parameter with increasing degree of order during the bcc-to-CsCl-type ordering transformation is explained in terms of an increase in atom-jump correlation with the onset of long-range order. © 1970 The American Physical Society.