The development of the microstructure of simple glass-ceramics, of molar compositions 70SiO₂·30Li₂O and 69SiO₂·30Li₂O·1P₂O₅, has been studied by the techniques of transmission electron microscopy, optical microscopy, X-ray powder diffraction and electrical loss measurements. Surface nucleation of lithium disilicate crystals occurred in all samples, and internal nucleation occurred in samples of the first glass treated for 1 h at 450, 475 or 500°C before crystallization at 750°C. Internal nucleation always occurred in samples of the glass containing P₂O₅ which were crystallized at 750°C, but the grain size of the crystalline product depended strongly on the temperature of the previous lower temperature heat treatment (the “nucleation stage”). The optimum nucleation range was close to 500°C for 1 h heat treatments. The results suggest that glass-in-glass phase separation, which was observed in both glasses, did not promote crystal nucleation directly, but effects due to an influence of phase separation on crystal growth rates are suggested. Possible reasons for the action of P₂O₅ as a nucleation catalyst are discussed, but firm conclusions cannot be reached on the present evidence.