Experiments with two massive deuterium-loaded palladium samples designed to search for deuteron-deuteron (d-d) fusion during thermal degassing are described. In the heavier of the two samples, which has a total mass of ∼0.5 kg, during deuterium expulsion from the metal, a significant neutron excess count rate was detected by two independent NE-213 scintillation neutron detectors. The maximum time-dependent excess count rate corresponds to a d-d reaction rate of (3 ± 1) × 10−25 per deuteron pair per second. From detector pulse high spectra, the energy of the neutrons is determined to be ∼2.5 MeV, as expected for d-d fusion neutrons.