Euro VI standards for heavy duty vehicles require the use of a DPF in order to comply with the particulate matter emission limit. Although passive regeneration of soot by NO2, promoted by a DOC located upstream the DPF, is preferred, the use of an active regeneration might be required whenever the soot mass trapped in the DPF increases. Some manufacturers made the choice of having a fuel injection in the exhaust system in order to generate an exothermic reaction in the DOC that helps to regenerate the particulate filter. This dedicated circuit avoids the use of in-cylinder post-injection which may induce oil dilution by diesel.The DPF regeneration is efficient and the DOC works durably if the exhaust diesel spray is completely vaporized before entering the DOC and is thoroughly mixed with the exhaust gases. However, ensuring complete evaporation and an optimum mixture distribution in the exhaust line is challenging.Optimization of the mixture in the exhaust line is an issue widely demanded by OEM's, whether as here in the DOC-DPF coupling to regenerate the DPF or in the distribution of AdBlue for SCR aftertreatment system. The objective is to experimentally assess the quality of the mixture between exhaust gases and exhaust diesel spray. The originality of the approach presented here is to directly visualize the HC concentration profile entering the DOC on a production exhaust line.In order to view any heterogeneities of concentration, characterizations by LIF (Laser Induced Fluorescence) were performed on the engine test bench with an exhaust line equipped with optical accesses. These experiments, carried out on heavy duty engine, made it possible to qualify the distribution of hydrocarbons in the DOC. The results have shown the potential and benefits of this type of visualization which helps to design efficient and durable after-treatment systems.