Co : Al2O3 nanocermets are synthesized by co-reducing Co2+-cations dispersed in a mesoporous AlO(OH) . alphaH(2)O matrix (amorphous) in a pure H-2 atmosphere at 850-1150 K. The dispersed Co2+-cations in pores co-reduce to separated Co-particles of controlled size, as small as 50 nm, encapsulated in thin Al2O3 films, which are formed in-situ by molecular decomposition of the matrix, 2AlO(OH) . alphaH(2)O --> Al2O3 + (2 alpha + 1)H2O. The Al2O3 film which coats Co-particles has an amorphous structure. This is possible only if it is extremely thin limited to a thickness of t < 2r(c), with r(c) similar to 1.9 nm the critical size of its nucleation and growth into a stable crystallite. The thin Al2O3 surface film supports the formation and existence of Co-particles in a modified FCC or BCC crystal structure. As a result, unusually, large crystallites of an average 100 nm diameter could be observed. Normally, such large particles of pure Co-metal exist in an HCP structure which undergoes a reversible martensitic transformation to FCC structure at 695 K. The results are analyzed and discussed in terms of microstructure, x-ray diffraction and XPS studies of nanocermets prepared under selected conditions.