The flow in the wake and near-wake regions of individual Taylor bubbles rising through stagnant and co-current vertical columns of Newtonian liquids was studied, employing simultaneously particle image velocimetry (PIV) and pulsed shadowgraphy techniques (PST). Experiments were made with water and aqueous glycerol solutions covering a wide range of viscosities (1 x 10(-3)Pa s < mu < 1.5 Pa s), in an acrylic column of 32 mm, ID. Different wake structures (laminar, transitional and turbulent) are identified, in both stagnant and co-current flow conditions. In stagnant liquids, the wake flow pattern is only dependent on the dimensionless group N-f. The different types of wakes obtained are in accordance with the critical Nf numbers proposed in previous works. For co-current flow conditions, the flow patterns in the wake depend on the Reynolds number based on the relative (to the bubble) average velocity of the upward liquid flow, the laminar-transitional and transitional-turbulent limits being for the first time experimentally determined. The wake flow patterns are quantified by means of instantaneous and average flow fields. Values for the wake length and wake volume are also presented and compare well with correlations found in literature. Study of the flow in the near-wake zone enabled determination of the distance needed to recover the undisturbed liquid velocity profile. The detailed study of the flow in the wake and near-wake regions is an important contribution to better understanding the interaction and coalescence mechanisms between Taylor bubbles. The data reported are relevant to the validation of numerical simulation codes in the vertical slug flow regime. (c) 2006 Elsevier Ltd. All rights reserved.