Interdiffusion of heterostructures based on the GaSb/AlSb material system is investigated by means of bright-field transmission electron microscopy (TEM). Samples grown by metal organic molecular beam epitaxy (MOMBE) are subjected to annealing experiments under both Sb-rich and Ga-rich conditions. Changes of concentration profiles of the group-III elements are determined by evaluating thickness fringes of cleaved 90° wedge specimens on a manometer scale. Interdiffusion is found to be significantly slower under Ga-rich conditions than under Sb-rich conditions and can be described numerically by an interdiffusion coefficient with non-linear concentration dependency. The experimental results are interpreted in terms of a diffusion mechanism by group-III vacancies.