A novel atomic layer deposition (ALD) process was developed for fabricating quaternary cobalt oxide (La_1−xSr_x)CoO_3−δ thin films having the eye on future applications of such films in e.g. solid oxide fuel cell cathodes, oxygen separation membranes or thermocouples. The deposition parameters and the conditions of a subsequent annealing step were systematically investigated, and using the thus optimized parameters the cation stoichiometry in the films could be accurately tuned. The most detailed study was conducted for x = 0.7, i.e. the composition with the highest application potential within the (La_1−xSr_x)CoO_3−δ system.