In this study the effect of the gas-phase density on the process of bubble formation at a single orifice in a two-dimensional gas-fluidized bed has been studied experimentally and theoretically. Specifically, a detailed comparison between experimentally observed and theoretically calculated bubble growth curves has been made in the case where the density of the gas injected through the orifice (He and SF6) differs significantly from the density of the primary fluidizing agent (air). The calculations have been carried out using an earlier developed, first principles hydrodynamic model of gas-fluidized beds which has been extended with a species conservation equation to calculate the composition of the fluidizing gas in the vicinity of the evolving bubbles. Besides, the present experimental and theoretical results were compared with predictions obtained from adapted versions of approximate bubble formation models previously reported in the literature. The advanced hydrodynamic model appears to predict the experimentally observed diameters satisfactorily. In addition, the model correctly predicts the effect of the gas-phase density on the experimentally observed bubble growth. This effect can be explained satisfactorily in terms of the dependence of the interphase momentum transfer coefficient on gas-phase density. Finally, calculations with a three-dimensional version of our hydrodynamic model have been carried out to account for the effect of the front and back wall of the pseudo two-dimensional gas-fluidized bed used in our experiments. Our preliminary computational results indicate that the magnitude of the wall effect strongly depends on the boundary condition enforced for the gas-solid dispersion at these walls. In the case that the no-slip boundary condition was enforced in the calculations for the solid phase, the wall effect was significant and a considerable deviation between computed and experimentally observed bubble growth curves was found. However, when a more realistic partial slip boundary condition for the solid phase was implemented the agreement between theory and experiment could be improved by altering the slip parameter in the partial slip boundary condition expression.