The mixing process is studied in grid-generated turbulent flow for single- and bubbly two-phase flow systems. Concentration and mixing characteristics in the liquid phase are measured with the aid of a PLIF/PLIF arrangement. A nearly isotropic turbulent flow field is generated at the center of the vertical pipe by using a honeycomb, three grids and a contraction. In two-phase flow experiments, air bubbles were injected into the flow from a rectangular grid, with mesh size M = 6 mm, which is placed midway between two circular grids each with a mesh size of M = 2 mm. For single-phase flow, the normalized mean concentration cross-stream profiles have rather similar Gaussian shapes, and the cross-stream profiles of the normalized root-mean-square (RMS) values of concentration were found to be quite similar. Cross-stream profiles of the mean concentration, for bubbly two-phase flow, were also found to be quite similar, but they did not have the Gaussian shape of the profiles for single-phase flow. Almost self-similar behavior was also found for the RMS values of the concentration in two-phase systems. The turbulent diffusion coefficient in the liquid phase was also calculated. At the center of the plume, the flow was found to have a periodic coherent structure, probably of vortex shedding character. Observations showed that the period of oscillation is higher in the case of two-phase flow than in single-phase flow.