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. Detailed experiments including differential pressure and void fraction measurements, are carried out on a 10 mm-diameter setup. Based on the results obtained, it is shown that existing models are not appropriate for small diameter airlifts, particularly because they overpredict the frictional pressure drop in slug flow. A new steady state airlift model is proposed. The pressure gradient in the riser is predicted by a combination of specific models describing slug and churn flow. These models are based on the available literature on two-phase flow. The particular structure of slug flow is accounted for by a cellular model. The model proposed represents an accurate analysis tool for the design of small diameter (up to 40 mm), tall (length-to-diameter ratio greater than 250) airlifts.