In an airlift pumping process, air is injected into the pipe containing the fluid to be transferred. Small diameter airlift pumps are, in particular, used for corrosive or radioactive liquids. However, for certain combinations of the geometrical parameters and air flow rate, they may become unstable. In this case, the flow at the riser outlet pulsates strongly, which cannot be accepted for many applications.An airlift pump involves three different regions, e.g. a single phase liquid flow and a separate single phase gas flow upstream of the air injection device and a two-phase flow downstream. The instabilities are due to density wave oscillations in the two-phase flow. Depending on the liquid flow inertia, friction effects and gas flow compressibility, the density waves are sustained or not.The present study is based upon a detailed description of the steady state flow in a small diameter airlift pump. A linear stability analysis is performed and assessed against an extensive set of experimental data. Both the experimental and analytical results show that the influencing parameters have complex effects and strongly interact : the same variation of a parameter may have opposite effects, i.e. stabilizing or destabilizing, depending on the values of the other parameters. The effect of the compressibility of the gas flow between the regulating valve and the air-injection device is shown to be very important.The analysis presented leads to a numerical model that can be considered as a practical tool for airlift performance and stability analysis.