The transport processes that are involved in the mixing of two gases in a T-junction mixer are investigated. The turbulent flow field is calculated for the T-junction with the k- turbulence model by FLOW3D. In the mathematical model the transport of species is described with a mixture fraction variable for the average mass fraction and the variance of the mixture fraction for the temporal fluctuations. The results obtained by numerical simulations are verified in a well-defined experiment. The velocity as well as the concentration field are measured in several types of T-junctions. Comparison of the predicted and measured average concentration fields show good agreement if the Schmidt number for turbulent diffusion is taken as 0.2. Temporal concentration fluctuations are calculated and found to be of equal magnitude as spatial fluctuations. Good mixing is obtained in a T-junction if the branch inlet flow is designed to penetrate to the opposite tube wall in the mixer.