Oriented aggregation is a special case of aggregation ill which nanocrystals self-assemble and form new secondary single crystals. This process has been suggested to proceed via all intermediate state known as the mesocrystal, ill which the nanocrystals have parallel crystallographic alignment but are spatially separated. We present the first direct observations of mesocrystals with size and shape similar to product oriented aggregates by employing cryo-TEN to directly image the particles ill aqueous suspension. The cryo-TEM images reveal that mesocrystals not only form but also transform to the final single crystal product while ill the dispersed state. Further. high-resolution cryo-TEM Images demonstrate that the mesocrystals are composed of spatially separated and crystallographically aligned nanocrystals.