Cobalt ferrite (CoFe2O4) was reduced in CO-CO2 gas mixtures at 1173 K at total pressure between 6.6 × 103 and 3.3 × 104 Pa, and at CO/CO2 ratios between 2.9 and 11.8. The reduction led to the formation of metal whiskers. The experiments and analysis emphasized the behavior of the whisker diameter during reduction. Impurities such as calcium and potassium stimulate metal nucleation but appear to inhibit catalysis of gas reactions at the metal/gas/oxide triple junction. The steady state whisker diameter was found to be inversely proportional to the total gas pressure at constant CO/CO2 ratio. A new model is proposed to explain whisker development. It considers metal/oxide interface diffusion coupled with a metal/oxide/gas triple junction reaction at the whisker base as the process determining the whisker diameter.