Individual bubbles rising through suspensions of spherical particles neutrally buoyant in viscous Newtonian liquids are tracked with real time radiography. The effects of the containing cylinder, suspended particle, and bubble sizes are reported for particle volume fractions ranging from 0.20 to 0.50. In the most concentrated suspensions, the effect of the bottom surface is felt by the bubble much farther into the suspension than would be true for a single-phase Newtonian liquid. Corrected for wall effects, the apparent viscosity felt by the bubbles is independent of the relative size of the bubble to the particle over a wide range of parameters, and is approximately linear with the suspended particle volume fraction, in contrast to the exponential dependence derived in conventional rheometry. Finally, in the most concentrated suspensions, the bubble deforms to a prolate spheroidal shape despite the lack of significant inertial effects or a viscoelastic suspending liquid. (c) 2008 American Institute of Chemical Engineers.