A theoretical model for tubular film blowing, based on treating the bubble as ＇＇quasi-cylindrical＇＇ at each point and incorporating a deformation-thinning viscosity equation, explained all of the essential features of the experimental data. The viscosity function required to fit the blown film data was in plausible agreement with viscosity data (complex viscosity data) measured independently. Under most conditions the model predicts an ＇＇intuitive＇＇ effect of inflation pressure on blow-up ratio; that is, increasing the pressure causes the final radius (the blow-up ratio) to increase. The predicted passage from an ＇＇intuitive＇＇ regime to a ＇＇counterintuitive＇＇ regime, which occurs at high blow-up ratios, results from the thinning behavior of the viscosity model. comparisons with the earlier model of Pearson and Petrie are also made. This model, based on thin-shell theory, disagrees with the data in several essential ways: it predicts counterintuitive results as regards the effect of inflation pressure and melt viscosity on bubble radius. These effects came about from an axial curvature term in thin-shell theory. The differences between the two models are discussed in detail.