Glasses have been obtained from mixtures of porphyric sands and dolomite. The influence of changes in the TiO2 content(0-6%) on the glass structure and the formation of crystalline phases on reheating has been studied. The experimental results suggest that in the studied system TiO2 promotes glass-in-glass phase separation and plays the role of a nucleating agent. The activation energy for crystal growth, E(C) = 486 kJ mol(-1), and the Avrami parameter (m = 1) have been evaluated by means of thermoanalytical techniques in the case of the base glass (no TiO2 added). The value of the Avrami parameter (tit = 1) agrees well with SEM observations of dendritic crystal growth from surface nuclei. In the other glasses, lath-like crystals were observed, the microstructure being finer the greater the TiO2%. The first addition of TiO2 (2%) gives, on quenching, a partially devitrified product; subsequent additions of titania, surprisingly, allow glasses to be formed more easily. The experimental results suggest that Na2O and K2O, present in the porphyric sands and therefore in the glasses (to a total amount of approximate to 5.6%), segregate preferentially into the titania-rich phase with respect to MgO. Taking into account that Na2O and K2O are useful in lowering the liquidus temperature of glasses but are known to have a negative effect on the mechanical properties, this can be important in the production of glass ceramics.