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The influence of the introduction of ZrO2 in concentrations of up to 5 mol% into a 30K2O-70SiO2 base glass composition has been investigated with the aid of differential thermal analysis (DTA), FT-IR spectroscopy, as well as chemical durability tests (pH, conductivity and AES-Inductively Coupled Plasma measurements) and refraction index determination. Several differences have been found by comparison with similar compositions belonging to the Li2O—ZrO2—SiO2 system. The higher ionic radius and the lower field strength of K+ with respect to Li+ is responsible for higher stability towards crystallisation and lower chemical durability. Moreover, even though Zr4+ increases glass polymerisation (higher glass transition temperature, chemical durability and refraction index), the presence of high water content as highlighted by spectroscopy measurements, seems to weaken the glass structure.