Abstract The influence of particle size (dP) and solid concentration (Cs) on the hydrodynamics of a slurry bubble column were studied experimentally using ultrafast electron beam X-ray tomography. Experiments were carried out for dP=50–150μm at Cs=0–0.20 and superficial gas velocity UG=0.02–0.05m/s in a slurry bubble column of diameter DT=0.07m. The real time 3D gas flow structure, average bubble diameter and averaged gas hold-up were found to be independent of dP and Cs at small dP≤100μm and low Cs≤0.03, however, at large dP and high Cs, strong dependency on dP and Cs were observed. Corresponding three-phase simulations were also performed using the Euler–Euler multi-fluid model implemented in CFX 14. The bubble size distribution was calculated using the MUSIG model. The predicted gas hold-up and bubble size distribution also shows dependence on dP and agreed quite well with the measurements and previous studies.