The action of water solutions of dimethyl formamide on Dinitrile A fibers was studied using the stress‐relaxation technique. After prior conditioning and stress relaxation in water, the addition of dimethyl formamide‐water solutions produced an exponential stress relaxation. It was found that the rate and extent of stress relaxation in dimethyl formamide‐water solutions is dependent on the dimethyl formamide concentration. The rate of stress relaxation in dimethyl formamide‐water solutions is also dependent on the cross‐sectional area and the state of orientation of the fiber. These data can best be explained by postulating that the rate of diffusion of the dimethyl formamide‐water solution into the fiber controls the rate of stress relaxation. The diffusion coefficient is found to be concentration dependent, and the diffusion process has an activation energy of 14 kcal.