A diffuser for aiding in the vaporization and mixing of an injected reactant with the exhaust gas feed stream of an exhaust gas aftertreatment system is disclosed. The diffuser is disposed within an exhaust gas conduit of the exhaust gas aftertreatment system and has an impingement surface and a baffle plate disposed downstream of and adjacent thereto. The baffle plate extends outwardly from the diffuser, to terminate at a baffle plate tip that is located intermediate of the diffuser and an inner wall of the exhaust gas conduit. The baffle plate is operable to trap a portion of an exhaust gas feed stream, and a reactant resident therein, to increase the vaporization and residence time of the reactant in the exhaust gas feed stream.