Parallel plate rheometry and tensile creep measurements were used to measure the equilibrium viscosity as a function of temperature of the bulk metallic glass PdNiP in the undercooled liquid state. The pronounced stability of this easy glass forming system against crystallization allows to cover a temperature range of about 110 K for viscosity measurements on the metastable equilibrium liquid state. The applied complementary methods yielded continuous values for viscosity in the range from 10 14 to 10 5 Pa.s. This exceptional wide region of experimentally determined values was found to be well suited to compare different theories and approximations for the temperature dependence of the viscosity known from the literature. It turnes out, that the results can be described best by free-volume theory.