Density, viscosity, and electrical conductance of concentrated H2SO4/H2O systems (above 66 wt% H2SO4) were measured in the subambient temperature region. The lowest temperature of measurement was 198 K. Density varied linearly with temperature. Viscosity and conductance data were fitted to the Vogel-Tammann-Fulcher (VTF) equation and also to the power-law equation, the latter being a better functional form to describe the temperature dependence of transport properties. Concentration dependences of viscosity, conductance, expansivity, and ideal glass transition temperature indicate the occurrence of structural changes in the H2SO4/H2O system in the regions 83-85 and 92-94 wt % H2SO4 which are attributed to the formations of H2SO4 H2O (at 84.48 wt% H2SO4) and the eutectic between H2SO4 . H2O and H2O (at 93.8 wt % H2SO4), respectively. Above similar to 75 wt % H2SO4 a direct correlation exists between the viscosity-concentration curve and the phase diagram of the H2SO4/H2O system.