There is a growing market for polymeric gas separation membranes for applications such as air separation and carbon dioxide capture. One of the key properties dominating transport is the free volume between atoms, allowing gas diffusion. However, thin films, in particular, undergo aging, decreasing free volume, and, hence, performance with time. We have measured the change in free volume during aging of thin films of a polymer of intrinsic microporosity (PIM-1) by depth-resolved positron annihilation lifetime spectroscopy. For films with thickness, d, smaller than 1 mu m, aging is nearly complete after 3 months, whereas for films with d>1 mu m aging continues even after several months. Aging is thickness-and time-dependent and the free volume diffuses through the film to the free surface.