The experimental heat capacity of Na8Si46 has been determined from 35–300 K, and its lattice parameters have been measured over the range 100–330 K. The experimental heat capacity and the thermal-expansion coefficient are compared with theoretical lattice-dynamical calculations for Na8Si46. The latter accurately reproduce the experimental thermal expansion and also give the first reliable assignment of the vibrational spectrum of this material, as judged by comparison of the calculated and experimental heat capacities. In addition, the theoretical results allow a calculation of the Grüneisen parameter of Na8Si46, which shows enhanced anharmonicity at low temperatures.