The world population is ageing. Led by diminishing birth rates, increased material prosperity, improved working and environmental conditions, better nutrition, and medical advances, this growing demographic change towards an ageing population poses challenges to national health and social security systems. For the individuals themselves, however, increasing life expectancy opens up opportunities to start a new phase in life after retirement. One practice of particular importance for ageing populations is transnational mobility. Older adults can, for example, travel the world, move to another country or spend part of the year in a place of their choice. To provide a better understanding of such transnational mobilities in old age, a mixedmethods research project was conducted in the framework of the NCCR-on the move. This project produced quantitative data through two Transnational-Ageing Surveys, and a qualitative study conducted in Switzerland and Spain. The mixed-methods project aimed to collect complementary data on transnational practices developed by ageing populations with a connection to Switzerland. This codebook summarizes data collected in the qualitative part of the mixedmethods research project. It presents information related to the interview modes and contexts; the socio-demographic characteristics of participants; the postretirement mobilities observed in the field; and cross-border financial transfers.