The influence of the interfacial tension on the composition range within which fully co-continuous polymer blend structures can exist is studied for different blends with selected matrix viscosities and viscosity ratios. The critical composition for full co-continuity is found to increase with increasing interfacial tension, narrowing the composition range. The effect of the interfacial tension on the critical composition is composed of two counteracting effects, i.e. the stability of the co-continuous morphology and the phase dimensions. The latter effect is smaller than the former. The experimental results can quantitatively be predicted by a model published earlier, provided the phase dimensions are measured separately.