We have experimentally studied the diffusion thermopower of a serial double quantum dot, defined electrostatically in a GaAs/AlGaAs heterostructure. We present the thermopower stability diagram for a temperature difference Delta T = (20 +/- 10) mK across the device and find a maximum thermovoltage signal of several mu V in the vicinity of the triple points. Along a constant energy axis in this regime, the data show a characteristic pattern which is in agreement with Mott's relation and can be well understood within a model of sequential transport.