[en] 1 - Description of problem or function: Format: Standard CCCC-III interface format. Number of groups: 50; Nuclides: Delayed neutron yields and spectra are provided for the seven isotopes Th-232, U-233, U-235, U-238, Pu-239, Pu-240, Pu-241. Origin: 90 materials of ENDF/B-IV plus the copper isotopes and lumped fission products of ENDF/B-III (total of 101 nuclides). Weighting spectrum: Thermal-1/E-fission. LIB-IV is a library of multigroup constants in standard CCCC-III interface format intended for (1) fast reactor core design, (2) CSEWG data testing and (3) processing code comparison. It contains cross sections and self-shielding factors for all 90 materials of ENDF/B-IV plus the copper isotopes and lumped fission products of ENDF/B-III, making a total of 101 nuclides. Data is available at zero degrees Kelvin and infinite dilution, plus three additional temperatures (300 K, 900 K, 2100 K) and up to six additional values of sigma-zero. Delayed neutron yields and spectra are provided for the seven isotopes Th-232, U-233, U-235, U-238, Pu-239, Pu-240, Pu-241. A 50-group structure is used, with a thermal-1/E-fission weight- ing function. P0 through P3 matrices are provided when data permits. 2 - Method of solution: The library was generated from ENDF/B-IV data using the codes MINX (2) (PSR-0105), and NJOY (3) (PSR-0171)