Binary mixtures of ammonium perfluorooctanoate with ammonium decanoate have been examined by a combination of H-1 and F-19 NMR spectroscopy over a range of fluorocarbon:hydrocarbon ratios. From chemical shift data obtained on the mixtures it has been possible to elucidate the micelle composition as a function of total surface active agent concentration. The experimental results can be described approximately by a regular solution model, and show nonideal, unfavored mixing of the hydrocarbon and fluorocarbon species in the micelle.