Tourism contributes 8% to global carbon emissions. Yet, only 10% of air passengers purchase voluntary carbon offsets. We test the effectiveness of different communication messages to increase voluntary purchasing of carbon offsets by air passengers. Results of a discrete choice experiment indicate that air passengers prefer carbon offset schemes that fund local programs (as opposed to international programs), that are effective in mitigating emissions, and are accredited. The ability to choose the specific offsetting program to be funded is not important. The willingness-to-pay for carbon offsets when booking for a group is lower than when booking an individual flight for oneself. Three market segments with distinct preferences exists. Segments also differ in key personal characteristics, including age, employment status, frequent flyer membership, and flight behaviour, making them actionable target segments for aviation carbon offsetting.