Turbulent mixing of two isothermal air streams in a T-junction of square ducts are investigated. Three dimensional velocity fields and turbulent kinetic energy are measured with stereoscopic Particle Image Velocimetry (PIV). The concentration field is obtained with a planar Mie scattering technique using the stereoscopic PIV setup. The concentration measurement method is developed in the present study and the
accuracy of the technique is investigated. The resulting data are two dimensional concentration fields taken at 4Hz. The combination of velocity, turbulence and concentration fields give valuable insight into the mixing process, e.g. by showing large scale flow instabilities. The present technique is well suited for easy testing of mixing devices and for validation of computational models.