A slip analysis has been developed to calculate the slip velocities from the capillary rheometry data for a polydimethylsiloxane (PDMS) gum, and two silicon rubber compounds. The analysis generalises the classical Mooney method [J. Rheol. 2 (1931) 210-222] to incorporate the influence of die geometry on the slip behaviour. For the PDMS gum, no slippage was observed below a stress level of about 60 kPa, and there was a jump in the slip velocity at a stress level of about 80 kPa. The complex theological behaviour of the rubbers meant that the analysis was only applicable at higher stress levels. For each material, a reasonable fit to the slip velocity was obtained using a generalised Navier slip law, which can easily be implemented into computational fluid dynamic simulations. Ultimately, a more realistic slip law is required to model the observed flow behaviour correctly. (c) 2005 Published by Elsevier BV.