An ordered mixed structure of c(3 x 2) is formed for a (NO + CO) coadsorption layer. The c(3 X 2) islands are considered to consist of equimolar NO and CO. The local fractional coverage (theta(NO) + theta(CO)) in the domain is estimated to be 0.33. Explosive production of CO2 takes place in the c(3 X 2) islands. The vacancy requirement model is considered to be valid for the autocatalytic reaction. Since the reaction is not accompanied with any substrate reconstruction, the autocatalytic behaviour is attributed only to the formation of mixed islands. The desorption of N-2 follows, however, the second-order kinetics on Pd(100). As a result of the competition between NO and CO for the surface electrons, the CO-metal bond is weakened by the coadsorbed NO, which influences the explosive reaction. On the other hand, strengthening of the NO-metal bond is observed. When NO is in excess of CO, a p(3 x 2) structure coexists with the c(3 X 2) structure. The local coverage in the p(3 x 2) islands is estimated to be 0.33. In this coverage region, another path for the CO2 production is available.