The mixing of a tracer in a reactor composed of two opposed jets perpendicular to a main stream is studied both experimentally and theoretically. For the experimental conditions, the gas flow was in a transitional state between laminar and turbulent due to the interaction of the mixed streams. Both the measurements and the mathematical model focus on the fluid flow (gas velocity) and the dispersion of a nonreacting species (tracer concentration) injected through one of the reactor streams. Two models are tested against experimental data: a laminar flow model (giving a time-dependent solution) and a standard k-epsilon turbulent flow model. These two models give quite different results, both qualitatively and quantitatively. This study demonstrates the importance of the selection of an appropriate mathematical model and computational domain.