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In the glass industry, nucleation agents are broadly used to obtain homogeneously dispersed and nanograined crystals during heat treatment leading to glass ceramics. In this paper, the role of zirconia and titania used as nucleation agents in a lithium alumosilicate glass is studied on the nanoscale using cutting edge analytical and imaging techniques performed in the transmission electron microscope. It is found that the formation of ZrTiO4 nanocrystals upon heat treatment is accompanied by the formation of a diffusion barrier surrounding these precipitates. At the interface between ZrTiO4 nanocrystals and the residual glass, an enrichment of alumina is observed, making the glass network more rigid and eventually preventing further growth or ripening of the nanosized crystals. Within the compositional gradient formed around the homogeneously distributed ZrTiO4 crystallites, nucleation of the secondary crystalline phase, a lithium alumosilicate high-quartz solid solution, is likely to be much facilitated rather than the never doubtlessly reported on epitaxial overgrowth assumed to act so far.