We report the effects of rare earth Gd doping on the spin entropy in Ca 3 Co 4 O 9+δ . Magnetothermopower is found to be strongly magnetic-field dependent and increases upon Gd doping, revealing that Gd doping can improve the spin entropy of Ca 3 Co 4 O 9+δ . X-ray photoemission spectroscopy (XPS) results allow us to determine the decrease in Co 4+ concentration induced by Gd doping. Our results confirm that the spin-entropy enhancement originates from the decrease of Co 4+ concentration. The present study suggests that we can improve the spin entropy by suppressing Co 4+ concentration in layered cobalt oxides.