Ab initio harmonic force fields were computed using the Becke3-Lee-Yang-Parr hybrid density functional with the standard 6-31G* basis set for dimethylnitramine, for dimethylamino radical, and for the transition structure for five-center elimination of HONO from dimethylnitramine. These force fields were used to calculate the primary deuterium kinetic isotope effect for the HONO elimination, and the secondary isotope effect for the N-N bond homolysis of dimethylnitramine. The computed primary effect is k(H)/k(D6) = 4.21, and the computed secondary effect is k(H)/k(D6) = 1.40, both at 240 degrees C. Comparison with the experimentally observed isotope effect of 1.57 for decomposition of dimethylnitramine-d(6) in solution at 240 degrees C suggests a significant part of the observed effect is due to the secondary effect on N-N bond homolysis. Similar beta secondary deuterium isotope effects are expected for the N-NO2 cleavages that initiate the decompositions of the nitramine explosives HMX and RDX.