The chlorate-nitrous acid reaction was investigated in acid media, using a high concentration of reagents. It was followed via ultraviolet-visible light (UV-vis) spectroscopy and presented a complex behavior. The order of reagents, and the products formed by this reaction, are dependent on the concentration of reagents. For the high concentration set we used, the reaction has shown a first-order behavior for H+ and HNO2, and an order equal to 0.79 for chlorate. In this case, chlorine dioxide is formed. Moreover, chlorine dioxide starts to form only after all HNO2 has been consumed. This is the first time chlorine dioxide was observed to be formed by this reaction. Reduction of the concentration of reagents decreases the order of HNO2 to 0.91 and no chlorine dioxide is formed. An isosbestic point was found at 312 nm, which indicates a 1:1 ratio between nitrate ion and nitrous acid species. A model, with 14 independent species and 12 reactions is presented, which is able to simulate the experimental behavior for the low and high concentrations sets of reagents and it is a significant improvement in the understanding of the complex nitrogen and chlorine aqueous chemistry.