Single bubbles were generated from a capillary tube in quiescent water and the bubble formation process was studied in detail using high-speed video at two pressures, 1.38 and 0.93 kPa. The bubble equivalent spherical radius, r, was derived from the data sets of 100 bubbles: (1) the a and b semi-axes values for an ellipsoidal model and (2) a (more accurate) cylindrical integration of the bubble image in 1-pixel layers. The two methods were compared and showed a significant improvement with the more accurate integration approach. Based on the time trends in the derivatives of a and b, there growth phases were indentified and the different forces acting on the bubble were calculated. The analysis showed significant differences between the two cases, despite similar time from appearance to detachment. For the 0.93 kPa case, the bubble shape detachment is described by Cassini oval while for 1.38 kPa it is a lenmiscate. For the study conditions, the momentum force was negligible for both cases; however, the viscous drag force, added mass, and surface tension forces were not. The bubble eccentricity exhibited an oscillatory behaviour, which we propose arose from highly non-linear wave-phenomena. Finally, only the low-pressure case was in agreement with the predictions of Oguz and Prosperetti (1993); for flows less than the critical flow, the high pressure was not in agreement, indicating a smaller value for the transition to super-critical flow for the study conditions. (C) 2010 Elsevier Ltd. All rights reserved.