Initial aggregation kinetics for three particle sizes and broad range of Peclet numbers were investigated under turbulent conditions in stirred tank. This allowed us to observe the transition from diffusion-controlled to purely shear-induced aggregation. The evolution of the root-mean-square radius of gyration, zero-angle intensity of scattered light, and obscuration was obtained by small-angle static light scattering. For a given particle size the measured evolution of all integral quantities obtained for various volume average shear rates < G >, scales with a dimensionless time, tau(exp) = alpha(exp) x < G > x phi x t. The experimentally obtained aggregation efficiency alpha(exp), follows the power law alpha(exp) = Pe(-n), where Pe is the primary particle Peclet number. With increasing particle size a decrease in n is observed in accordance with theory and literature data. As previously predicted by population balance equation simulations three aggregation regimes were observed experimentally. (C) 2009 American Institute of Chemical Engineers AIChE J, 55: 3076-3087, 2009