This is a systematic investigation, using in situ synchrotron small-angle X-ray scattering, of the mechanisms of aggregation of sulfate zirconia sols under different preparation conditions. We have studied the effects of the variations of acid, water, and sulfate contents on the process of aggregate growth. These effects were investigated using open and sealed sample cells. It was observed that two clearly different mechanisms govern the aggregation at the beginning and at advanced stages of the process. In the early stages, small needle-shaped clusters are formed. They have a time independent cross-section and progressively grow by increasing their length. At advanced stages, the initially isolated needle-shaped clusters start to build up a 3D structure, their aggregation being governed by the classical mechanism of diffusion-limited cluster aggregation (DLCA). The experimental results also suggested an effect of sulfate concentration on the structure of the aggregates. Even though the structures of the final sols vary markedly due to differences in the chemical and physical preparation conditions, our experimental results suggest that a single and remarkably simple model explains the aggregation process for all the studied solutions.