Viscosity of Na/K tectosilicate glasses containing between 50 and 83 mol% of SiO2 increases non-linearly when K+ ions substitute Na+ ions in the liquid phase. The viscosity variations depend on chemical composition but cannot be reproduced using an ideal mixing model of the configurational entropy. Consequently, it appears that Na+ and K+ elements do not mix randomly in the studied aluminosilicate melts. We speculate that Na/K mixed tectosilicate glasses contain two sub-networks, one composed of the Si, Al, O and K atoms, and another of the Si, Al, O and Na, with a clustering behaviour of alkali cations into these different sub-networks.We propose to study various (KxNa(1-x)2O-Al2O3-SiO2 glasses by small angle neutron technique on D16 to obtain a detailed representation of their structure at the intermediate and long range order and to study its relationship to their thermal, rheological and mechanical properties.