Glass transition temperature (Tg) and Raman spectra were measured in the glass series of yMO1/2⋅xSnO2⋅(1-y-x) GeO2 (M: Li, K). The structural models were proposed based on the composition dependence of these properties on the assumption that the SnO6 octahedron in these glasses is exclusively connected to six GeO4 tetrahedra at all vertices (isolated SnO6).(1) LiO1/2-GeO2 binary glasses contain no Ge4O16 group. In the glass series with constant LiO1/2 content, GeO2 is substituted by SnO2 according to the reaction of type-I; one isolated GeO6 is substituted by one isolated SnO6.(2) In the glass series containing 20-35 mol% KO1/2, the substitution of SnO2 for GeO2 occur preferentially in order of the type-II, -I and-III; type-II and-III are the substitution of two isolated SnO6 for one Ge4O16 group and the formation of isolated SnO6 accompanied by the disappearance of two non-bridging oxygens, respectively. In the glass series containing 40-50mol% KO1/2, the substitution reaction type-III occurs in competition with I and II by the addition of small amount of SnO2 because of high content of non-bridging oxygens.(3) Based on these results, the amounts of all 6 coordinated Ge atoms and Ge4O16 group in KO1/2-GeO2 glasses were calculated. It was confirmed that fair part of 6 coordinated Ge atoms form Ge4O16 groups in these binary glasses containing 20-50mol% KO1/2.