Bubble behaviors are studied in a rectangular bubble column using particle image velocimetry (PIV). Bubbles rise in a chain through a stagnant liquid. As liquid viscosity reduces, bubble rising trajectory changes from one-dimensional to three-dimensional. This transition is due to different bubble wake structures. The bubble shapes also show dissimilar characteristics in liquid of different viscosities. The instantaneous liquid flow fields measured by PIV show the diversities as bubbles rise in different paths. Based on the experimental data obtained in this study, a correlation is proposed to calculate the terminal velocity of bubbles in a chain. (c) 2006 Elsevier B.V. All rights reserved.