The reactivity of NH2BH2 in the presence of ammonia-borane (AB) is investigated using ab initio CCSD(T) simulations to answer the following three questions: How do AB and NH2BH2 react? How do aminoborane species oligomerize apart from catalytic centers? Can the formation of experimentally observed products, especially cyclic N2B2H7-NH2BH3, be explained through the kinetics of NH2BH2 oligomerization in the presence of AB? AB is shown to react with NH2BH2 by the addition of NH3-BH3 across the N=B double bond, generating linear NH3BH2NH2BH3. This species decomposes by surmounting a reasonable barrier to produce two NH2BH2 and H-2. The generation of additional NH2BH2 from NH2BH2 and AB provides a pathway for autocatalytic NH2BH2 production. The important intermediates along the oligomerization pathway include cyclic (NH2BH2)(2) and linear NH3BH2NH2BH3, both of which have been observed experimentally. Calculations show cyclic N2B2H7-NH2BH3, an aminoborane analogue of ethylcyclobutane, to be the kinetically preferred stable intermediate resulting from oligomerization of free NH2BH2 over its isomers, cyclic B2N2H7-BH2NH3 and cyclotriborazane, cyclic (NH2BH2)(3). Simulations show cyclotriborazane formation to be kinetically slower than cyclic B2N2H7-NH2BH3 formation and imply that formation of the cyclic species cyclotriborazane and cyclopentaborazane may be catalyzed by binding of NH2BH2 to a catalytic metal center. Routes that may lead to larger noncyclic oligomers are suggested to be kinetically competitive. The highly reactive N=B double bonds of NH2BH2 are shown to be of central importance in understanding aminoborane oligomerization.