The viscosity of (hexane + perfluorohexane) mixtures has been measured at 298 K, 303 K and 308 K, over the whole composition range. The results show large negative deviations from the arithmetic mean of the viscosities of the pure components, reaching - 17% at x(perfluorohexane) = 0.7. To obtain molecular level insight into the behaviour of the system, all-atom molecular dynamics simulations have been performed and used to calculate the viscosities, radial distribution functions, and rotational relaxation times of the studied (hexane + perfluorohexane) mixtures. This is the first effort to assess the effect of mixing hydrogenated and fluorinated molecules in the viscosity. (C) 2013 Elsevier B.V. All rights reserved.