Earth-abundant Cu2ZnSn(S,Se)4 (CZTSSe) thin film photovoltaic absorber layers are fabricated by annealing Cu2ZnSnS4 (CZTS) nanoparticle thin films in a selenium rich atmosphere. Systematic variation of the selenization time (5, 10, 20 and 40 min) and temperature (450, 500, 550 and 600 °C) provides insight into the kinetics of the selenization process and in particular recrystallization and grain growth. Se–S anion exchange is found to follow Avrami׳s model in which the CZTS selenization is controlled by an irregular one-dimensional process limited by metal cation re-ordering and grain boundary migration. CZTSSe grain growth is observed to follow a normal relation with a grain growth exponent close to the ideal case of equiaxed grains and the grain boundary migration energy is calculated to be 85.38 kJ/mol. These selenization variables have a fundamental influence on the quality of the resulting CZTSSe thin film and consequently the device performance. A peak device solar energy conversion efficiency of 5.4% was obtained for selenization at 500 °C for 20 min. The device efficiency was found to be highly sensitive to these variables and it is critical to obtain an appropriate balance between grain growth and thin film quality.