Abstract The system GRACE generates elementary reaction networks for simple reactions including free radicals, ions and even active sites in heterogeneous catalysis, and also predicts the overall reaction rates and the product distributions in radical reactions in the gas phase where the Arrhenius parameters of elementary reactions are available. In this paper, A/GRACE (part A of GRACE) is explained, which prepares elementary reaction networks for radical reactions. The reactant and the product are represented by square symmetric matrices. The off-diagonal elements represent bond multiplicity, or the number of localized electrons, between corresponding atoms, whereas the diagonal elements imply the number of unshared electrons on radical atoms. A reacting system, an ensemble of the participating molecules, is composed of atom groups, each of which consists of a center atom (usually non-hydrogen) and attached hydrogen atom(s), if any. Firstly, all the possible changes in the atom groups are selected, and the permitted combinations are preapred bearing in mind the equivalent groups in the system and the restrictions optioned. The major restriction is the complexity, i.e., the number of ruptured or formed bonds. A direct sum of matrices are prepared from one of the combinations, and the elementary reaction is completed by setting the appropriate numerals, 1 or −1, to the outside elements. Secondly, the procedures are repeated until all the feasible networks are prepared. As an illustration the hydrogenation of ethylene over a heterogeneous catalyst has been cited.