Rapid magnetic resonance imaging is used to investigate the volume, shape and rise velocity of single isolated bubbles injected into incipiently fluidized beds as well as the particle velocity field surrounding these bubbles. The volume of gas injected and particle size are varied to investigate their effects on bubble behavior. Image processing is used to quantify data into plots which can be compared directly with predictions from analytical and numerical models. Results show that more injected gas leaks into the interstitial flow in beds of large particles than beds of small particles because of the higher permeability to gas flow. Bubbles develop from a taller shape with a large wake angle into a wider shape with a smaller wake angle as they develop from a spherical shape to a spherical cap shape as they rise. Bubble rise velocities for fully formed bubbles are consistent with correlations in the literature. (C) 2019 Elsevier Ltd. All rights reserved.