The solidarity contribution or 'tax' on airline tickets represents 70% of UNITAID's financial base and is complemented by multi-year budgetary contributions from a number of member countries.

The tax is applied to all flights departing from countries that impose it and is paid by passengers when purchasing their tickets, normally as an addition to existing airport taxes. Airlines then are responsible for declaring and collecting the levy. Passengers in transit are exempt, thus avoiding any further administrative burden for airports in participating countries. The solidarity levy fully respects countries' tax sovereignty.

For passengers, the cost of the air tax is very low compared to the total cost of a ticket; it can range from US$ 1 for economy-class tickets to US$ 10 and US$ 40 for business and first-class travel. Different rates can be set according to a country's level of development and there is an extra option to vary the charge according to the distance traveled. For example, some countries in Africa have chosen to impose the levy only on international flights or on business and first-class tickets.

As of September 2011, nine of UNITAID's 29 member countries were implementing the airline tax: Cameroon, Chile, Congo, France, Madagascar, Mali, Mauritius, Niger, and the Republic of Korea. Norway allocates part of its tax on CO₂ emissions from aviation fuel to UNITAID.