A technique for obtaining cross-section information from experimental isotopic concentration data has been applied to the data from Yankee Core I. The technique involves making a least-squares fit of appropriate transmutation equations to the data. The analysis covers two transmutation chains. One is the plutonium chain which begins with 238U and ends with americium and curium. The other is a rarer isotope chain which begins with 235U or 238U and ends with neptunium and plutonium. Values were obtained for 12 ratios determined by the leastsquares analysis (e.g., 25 = 0.2569 ± 0.0023 and 49 = 0.622 ± 0.082). In order to obtain information about the less abundant transuranium isotopes, 15 Yankee Core I samples were analyzed for their isotopic content. The ratios of effective cross sections were also deduced from the data.