Recently, a model for the growth mechanism of inorganic fullerene-like (IF) nanoparticles of MS2 (M = Mo, W) from the respective oxides was presented (Feldman, Y.; Frey, G. L,.; Homyonfer, M.; Lyakhovitskaya, V.; Margulis, L.; Cohen, H.; Hodes, G.; Hutchison, J. L.; Tenne, R. J. Am. Chem. Sec. 1996, 118, 5362). According to this model, sulfidization of oxide particles of <150 nm leads to the formation of a sulfide encapsulate with oxide core, which is progressively converted into a hollow IF nanoparticle. Using transmission electron microscopy, the sulfidization of a group of oxide nanoparticles is demonstrated step by step. This study provides direct evidence for the quasi spiral growth of the sulfide layers into the oxide nanoparticle core. However, the mechanism for the formation of the first closed sulfide layer which engulfs the oxide encapsulate remained a puzzle thus far. The analysis of the kinetics of simultaneous reduction and sulfidization of WO3 powders suggests the occurrence of a unique driving force for the fast growth of the first curved sulfide layer (001) around an oxide nanoparticle. According to the present model, a synergy between the reduction and sulfidization processes which occurs in a very narrow window of parameters leads to the formation of the first one or two closed sulfide layers. The present study is not limited to the sulfides. The formation of IF-WSe2 (Tsirlina, T.; Feldman, Y.; Homyonfer, M.; Sloan, J.; Hutchison, J. L.; Tenne, R. Fullerene Sci. Technol. 1998, 6, 157) is found to be consistent with the same kind of model.