This article presents the evolution of the catalyst and the beginning of carbon precipitation (germination) before the growth, carried out by a process already described[1], of carbon fibers from methane. The precursor of the catalyst is a film of ferric nitrate. The evolution of the catalyst particles is studied by interrupting the process at successive stages, quenching and observation under a TEM (Figs. 2 and 3). The mean diameter of the particles as well as the distribution of their sizes have been determined as a function of the highest temperature of treatment (Figs. 4–7). Average diameters increases with temperature; it is larger when heating has been carried out under pure hydrogen (Figs. 2, 4, and 6) than it is when the mixture H 2 CH 4 has been used (Figs. 3, 5, and 7). Size distributions at 950°C are showed on Fig. 8: curve C 1 after heating under the mixture, curve C 2 after heating under pure H 2. In the latter, the distribution is very sensitive to carbon: introduction of the mixture and heating to 1000°C leads to a redispersion (curve C 3). So, two antagonistic effects are evidenced: —Sintering particles induced by heating. —Redispersion and prevention of sintering by carbon coating; this redispersion is specially important when preheating has been carried out under pure hydrogen. Previous results have shown that no fibers are obtained from particles with diameters lower than 4 nm or higher than 14.5 nm and that catalytic lengthening of the fibers occurs between 1050 and 1100°C. The size effect on the melting point of the iron particles is derived here by a complete thermodynamic calculation taking into account the Young-Laplace law. It shows that the populations of particles that melt in the temperature range where the catalytic growth occurs correspond to the population observed in the fibers. We believe then that solely the melting of particles allows lengthening rate high enough to yield long carbon fibers before the catalyst particles are prisoned by pyrocarbon deposit. Figure 9 suggests a scheme of the mechanism leading from the precursor to fibers.