Physical parameters and structural characteristics are reported for the mixed alkali borosilicate glass system, xCs2O(100 - x)(MW) (0 ≤ x ≤ 9.66) where MW is the BNFL wasteform glass (10.29 mol% Li2O, 10.53 mol% Na2O, 18.57 mol% B2O3, 60.61 mol% SiO2). Glass density was found to increase with x whilst the glass transition temperature (Tg) decreased. The fraction of four-coordinated boron (N4) determined from 11B magic angle spinning nuclear magnetic resonance (MAS NMR) increased with x in a manner consistent with previous reports on the sodium-lithium-borosilicate system and contrary to the Dell model. 29Si MAS NMR was used to identify the fraction of Q4 (B) and Q3 units as a function of x. Raman spectra indicated the presence of reedmergnerite and danburite superstructural units. Mass loss measurements showed a non-linear variation with x, having a maximum between 7 and 8 mol% Cs2O and a correlation was found between the amount of Q3 units in the solid and the volatilization loss from the melt. © 2005 Elsevier B.V. All rights reserved.