A non-biological floe system consisting of clay minerals and an organic flocculating agent was used for the determination of mechanical stresses in turbulent multiphase flows. The floe disintegration kinetics i.e. the time and stress dependent size distribution of the flocs were observed with an optical in-line particle-system analyser and the raw data transformed. The concomitant measurement of floe disintegration in a reference system, a couette system, enabled the assignment of the effective mechanical stresses. The volumetric power input resulting of isothermal expansion of gas has been derived to be the governing factor for the effective shear stresses in bubble columns and airlift loop-reactors. Furthermore the effects of gas phase distribution (originated from the sparger design) and solids loading on shear stresses were determined for fluid systems with low viscosities. Especially the interactions between finely dispersed bubbles and particles seemed to cause a drastic change of turbulence.