This study deals with the effect of impurities on the free rise velocity of gas bubbles in three Newtonian and eight non-Newtonian polymeric liquids. Experimentally, the rise velocities of air bubbles in liquids which contain concentrations of a surfactant (sodium dodecyl sulphate) ranging from 0-500 ppm were measured. It is shown that the surfactant lowers the surface tension of the system and has an effect on the velocity of the bubbles, whose volume ranges from 1-1000 mu l. The experimental data were used to verify a recent theoretical approximation of the force on a bubble moving in a contaminated fluid described by the three-parameter Carreau model. An hypothesis is proposed to explain the experimental observation of a jump discontinuity in the log-log plot of bubble velocity versus bubble volume in the case of viscoelastic fluids. An equation for calculating for critical radius at which discontinuity occurs is given and is found to be in agreement with experimental data.