Electronic structure calculations have been performed to investigate the initial steps in the gas-phase decomposition of urea and urea nitrate. The most favorable decomposition pathway for an isolated urea molecule leads to HNCO and NH3. Gaseous urea nitrate formed by the association of urea and HNO3 has two isomeric forms, both of which are acid-base complexes stabilized by the hydrogen-bonding interactions involving the acidic proton of HNO3 and either the O or N atoms of urea, with binding energies (D-0 degrees, calculated at the G2M level with BSSE correction) of 13.7 and 8.3 kcal/mol, respectively, and with estimated standard enthalpies of formation (Delta(f)H298 degrees) of -102.3 and -97.1 kcal/mol, respectively. Both isomers can undergo relatively facile double proton transfer within cyclic hydrogen-bonded structures. In both cases, HNO3 plays a catalytic role for the (1,3) H-shifts in urea by acting as a donor of the first and an acceptor of the second protons transferred in a relay fashion. The double proton transfer in the carbonyl/hydrogen bond complex mediates the keto-enol tautomerization of urea, and in the other complex the result is the breakdown of the urea part to the HNCO and NH3 fragments. The enolic form of urea is not expected to accumulate in significant quantities due to its very fast conversion back to H2NC(O)NH2 which is barrierless in the presence of HNO3. The HNO3-catalyzed breakdown of urea to HNCO and NH3 is predicted to be the most favorable decomposition pathway for gaseous urea nitrate. Thus, HNCO + NH3 + HNO3 and their association products (e.g., ammonium nitrate and isocyanate) are expected to be the major initial products of the urea nitrate decomposition. This prediction is consistent with the experimental T-jump/FTIR data.