Many drugs are successfully loaded into preformed liposomes by using various gradients and transmembrane potential. Several experimental breakthroughs, however, have not been paralleled by theoretical understanding of the processes. Recently, we have developed a rigorous treatment of loading of weak acids and bases into liposomes. The model is based on equilibration of chemical potentials of permeable neutral species. Charged molecules are not allowed to permeate the membrane. Although this assumption is quite reasonable and experimental data fit the theoretical predictions rather well, we have extended the model of liposome loading. In the expanded model, terms which allow leakage of protons, buildup of the transmembrane pH gradient, an antiport exchange of various cations with protons, and leakage of other molecules from or into liposomes are added to the basic model.