The evolution of a continuous Fe(NO3)(3) catalyst film was observed by in situ annealing in a transmission electron microscope (TEM). Such catalysts are routinely used in the catalytic growth of carbon nanombes. The experiments reveal that crystalline particles form from the gel-like film already around 300 degreesC. At usual carbon nanotube growth temperatures of 700 degreesC, the particles are single crystalline with a log-normal size distribution. The observations furthermore show that despite the crystalline structure of the particles there is a high mobility. The experiments allow obtaining detailed information about the chemistry and the crystallinity of the catalyst film, which provides valuable information for the interpretation of the carbon nanotube growth by chemical vapor deposition.