Making metal and ceramic powders using aerosol synthesis from vapour precursors, either in a flame or hot-wall tube reactor, is the basis for producing many thousands of tons of powder on an annual basis. To properly study this system, we have designed and built a model reactor with sampling points at evenly spaced axial positions. This allows us to take snapshots of the aerosol population at many points within the reactor. Nucleation followed by a surface reaction produces a solid phase extremely rapidly, within 0.01 s under typical conditions. This is followed by a transient state where nucleation, surface reaction and coagulation all interact to produce a strongly bimodal size distribution. After nucleation is extinguished, the size distribution approaches the self-preserving limit as predicted for a coagulation-dominated process. The final structure is determined by the dominant sintering mechanism, which can be estimated from theory. The knowledge of this mechanism offers the possibility of selecting reactor conditions to produce powders with optimized properties.