Fuels are subjected to extreme conditions inside a fuel injector. In modern common rail diesel engines, fuel temperatures can reach 150°C and pressures can exceed 2500 bar inside the rail. Under such conditions the fluid physical properties of the fuel can differ substantially from ambient pressure and temperature and can impact the spray behavior and characteristics. Moreover, experimental determination of the fuel physical properties at these extreme conditions can be very difficult.Previously it has been shown that for pure components, all atom molecular simulations offer a reliable means to calculate the key physical properties (including transport properties, e.g., viscosity) at FIE representative conditions. In this study we extend the approach to calculate these properties of binary mixtures using atomistic molecular simulations. We modelled the diesel fuel using two diesel surrogates: a modern “GTL-like” diesel fuel surrogate, n-hexadecane, and a “conventional” diesel fuel surrogate n-decylbenzene and calculated viscosity and density for the respective pure components and their various binary mixtures.The molecular dynamics simulations provide insights into the mechanisms at an atomic level and can reliably calculate density and viscosity of the fuel at extreme conditions commonly found inside a fuel injector.