A combined particle image velocimetry (PIV) and particle image thermometry (PIT) method was applied to visualize and measure simultaneously the 2-D velocity and temperature fields in a crystallizer. The 2-D supersaturation field can be also determined before nucleation or seeding takes place. The hydrodynamic behavior in an internal loop airlift crystallizer was studied for different air flow rates, sparger types, and crystal holdups to get insight in the optimal process conditions for this crystallizer, i.e. uniform temperatures and particle concentration. The 2-D velocity and temperature gradients showed that in a 2 L crystallizer the mixing is sufficient to ensure uniform supersaturation profiles at an overall superficial air velocity higher than 2.3 mm/s. Suspension tests showed that a superficial air velocity of 7 mm/s was sufficient to avoid settling of the crystals. This air flow rate assured the lifting of the solids from the bottom of the crystallizer for crystals with a mean size of 1.5 mm and holdups up to 10 wt %. Both the circulation velocity and the crystal holdup which could be totally suspended increased with the air flow rate. For the ammonium sulfate-water system, the crystal growth behavior, which is also influenced by the mixing, was theoretically studied. The growth of the crystals in an airlift crystallizer behaves comparable to that in a suspension stirred crystallizer, and no evidence for mass transfer limitation of the crystal growth was found.