Superionic glasses were prepared in the systems (100 - x) (0.6Li(2)S . 0.4SiS(2)). zLi(x)MO(y)(Li(x)MO(y) = Li4SiO4, Li2SO4) by twin-roller rapid quenching. Glass transition temperatures (T-g) and crystallization temperatures (T-c) were determined for the glasses. The difference between T-g and T-c, which was one of the measures of glass stability against crystallization, was maxmized at the composition with 5 mol% Li4SiO4. The conductivity at 25 degrees C (sigma(25)) was as high as 10(-3) Scm(-1) for the glass with 5 mol% Li4SiO4. The doping of Li2SO4, however, did not improve the glass stability against crystallization nor sigma(25).