A method is described to estimate deuteron-on-deuterium breakup neutron distributions at 0 deg using deuteron bombardment of 3He. Breakup neutron distributions are modeled with the product of a Fermi-Dirac distribution and a cumulative logistic distribution function. Four measured breakup neutron distributions from 6.15- to 12.0-MeV deuterons on 3He are compared with 13 measured distributions from 6.83- to 11.03-MeV deuterons on deuterium. Model parameters that describe d-3He neutron distributions are used to estimate neutron distributions from 6- to 12-MeV deuterons on deuterium.