A numerical study on the scale-up behaviour of orthokinetic agglomeration in stirred tanks is presented. Large Eddy flow simulations were performed to obtain an accurate description of the turbulent flow encountered in stirred vessels, equipped with either a Rushton or a pitched blade turbine. Simultaneously, the convection-reaction equation for the particle number concentration is solved. Equal resolutions were used for the flow simulations and the particle concentration equation. Agglomeration was incorporated by making use of the nonlinear agglomeration model proposed by Mumtaz et al. [Trans. Inst. Chem. Eng. 75 (1997) 152]. Reactor performance for vessel sizes in the range of I to 10 000 1 was simulated. Three scale-up rules (viz. constant Re number, specific power input, and impeller tip speed) were investigated. It was found that impeller shape, vessel size, and Re number have a profound effect on reactor performance.