An internal friction peak induced by carbon diffusion has been observed in f. c. c. iron-nickel alloys. At a vibrational frequency of 1.4 cycles per sec., the peak occurs at about 500°K. This peak arises from the preferential rotation of the axes of carbon pairs existed in the specimen under the influence of an alternating load. These carbon pairs were formed by the interaction between the substitutional carbon atoms and their neighboring interstitial carbon atoms. Based upon this mechanism and considering the possible redistribution of the carbon atoms amongst the frozen-in vacancies in the specimen during the course of internal friction measurement, an equation bearing the quantitative relationship between the peak height and the carbon concentration has been derived. The energy of vacancy formation as well as the binding energy of the carbon pairs have been determined.The characteristics of the carbon diffusion peak in f. c. c. metals are: (1) The internal friction curve is not symmetric with respect to the peak position, the internal friction on the high temperature side being smaller than on the low temperature side. (2) The T′Qmax-1 product varies with the peak temperature T′; the lower the peak temperature, the higher the T′Qmax-1 product. (3) The peak temperature T′ drifts with the carbon concentration as well as the quench temperature of the specimen. Either an increase of the carbon concentration or a raise of the quench temperature tends to shift the peak position toward a lower temperature. These characteristics are interpreted in terms of the carbon pair theory as given in the text.