The literature on tubular film blowing is contradictory, even on such basic matters as to whether increasing inflation pressure increases or decreases the bubble radius. To provide a solid experimental base for physical understanding and theoretical modelling, detailed on-line measurements were made using three polyethylenes (a low density, a linear low density and a high density material, all of melt index 1.0). The measurements included blow-up ratio as a function of inflation pressure and take-up ratio; the melt temperature and the air flow rate were also varied. On-line radius, velocity and temperature measurements along the film were made in some runs. For the most part the data showed an ＇＇intuitive＇＇ effect of inflation pressure on blow-up ratio; that is, increasing the pressure caused the final radius (the blow-up ratio) to increase. However, at high blow-up ratios (typically values of 2 to 3, depending on the material and processing conditions), regimes having a ＇＇counterintuitive＇＇ relationship were observed in some cases. There were not substantial differences among the three materials except that the low density material was less prone to instabilities at high blow-up ratios.