Abstract: Detailed experiments are conducted to study hydrodynamic effects of two simultaneously released bubbles rising in viscous liquids. Different types of interactions are observed as a function of the liquid viscosities, leading to different bubble shapes, ranging from rigid spheres and spheroids to deformable spheroids. Bubble velocities are obtained by an automated smooth spline technique, which allows for an accurate calculation of the lift and drag forces. The results obtained for spherical bubbles are in agreement with predictions of Legendre et al. (J Fluid Mech 497:133–166, 2003). The observations of deformed bubbles show that a very small equilibrium distance can be established due to the induced torque arising from the deformation. In terms of the lateral interaction, different separation distances can be observed depending on the initial distance. For deformable bubbles, the results are limited to a qualitative analysis due to limitations of the processing technique to handle strong shape irregularities. Nevertheless, the observations reveal that the deformation plays an important role with respect to bubble interactions and path instability of which the latter can be triggered by the presence of other bubbles. Graphic abstract: [Figure not available: see fulltext.].