A constrained least-squares method is developed for deducing six-group delayed neutron energy spectra from composite spectra measured at six or more delay time intervals following fission. The constraining condition is chosen to yield stable solutions that also provide good fits to the measured spectra. The method is applied to previously measured composite spectra of 235U to obtain six-group delayed neutron energy spectra. The solutions are unique for a large range of constraint spectra. The dependence of the solutions on the choice of six-group parameters (ßj,λj) is also examined.