A free oscillation technique has been adopted to measure the dynamic storage and loss moduli of carbon black filled natural rubber materials. These tests are conducted with small oscillations that are superimposed on a range of tensile pre-strains. Tn addition, the effect of temperature on the dynamic moduli is measured as well as the effect of swelling the materials to various extents by liquids with a range of viscosity. It is observed that the dynamic storage and loss moduli do not depend strongly on the pre-strain at small pre-strains. At higher pre-strains there is a marked increase in both the storage and the loss moduli. An increase in temperature causes a dramatic reduction in both the storage and loss moduli. The dynamic behaviour of the filled rubbers when swollen can be approximately ascribed to the combined effects of a reduction in the modulus of the rubber matrix (caused by the swelling action) and a reduction in the effective volume fraction of the filler. The liquids used had a range of viscosity of more than a factor of a thousand. Despite this, the loss moduli of the swollen rubbers varied by only about a factor of two. This insensitivity could be understood in terms of a previously developed theory, based on free volume considerations.