The thermal fission yields of 43 mass chains for 233U, 235U, 239Pu, and 241Pu and the fast reactor fission yields of 45 mass chains for 235U and 239Pu have been measured with a relative standard deviation of 1 to 2%. The data were obtained mainly by measuring long-lived and stable fission products by the technique of isotope dilution mass spectrometry. This paper has two sections: the first reports the fission yields, the second describes the chemical analysis techniques for the fission products.