Low shear rate viscosity measurements of alternating copolymers of ethylene and tetrafluoroethylene (PETFE) melts have been performed at different temperatures by using the centrifuge ball viscometer. The viscosity eta (Pa s) could be obtained with an empirical relation, a(c) = (4.29 +/- 0.08) x 10(2)(eta V-proportional to)(0.856 +/- 0.004) , With a(c) and V-proportional to being, respectively, the acceleration (g = 9.8 m s(-2)) and the terminal velocity (m s(-1)) of a stainless steel ball of 1.58 mm diameter moving in the polymer melt, which was flame sealed in a horizontal cylindrical glass tube of 4.26 mm diameter. For the temperature dependence of the viscosity, an activation energy E(a) of 15.3 kcal mol(-1) was determined. The activation energy for PETFE is between 5.7-14.6 kcal mol(-1) for polyethylene (PE) and 18-36 kcal mol(-1) for tetrafluoroethylene (PTFE). A relationship between the zero shear rate viscosity eta(0) (Pa s) and the molecular weight M(w) (g mol(-1)) of PETFE copolymer melts at 280 degrees C [eta(0) = (1.23 +/- 0.02) x 10(-16)(M(w))(3.43 +/- 0.06)] should permit us to determine the molecular weight of PETFE with the centrifuge ball viscometer.