We have measured the thermoelectric power and resistivity of two different superconductor series as the doping level is increased into the overdoped regime where Tc decreases below its maximum value: Ba1-xKxBiO3 single crystals with K content up to x=0.55, and La2-xSrxCu1-yNiyO 4-δ ceramics with Sr content up to x= 0.57. For overdoped samples in both series, thermopowers that are small and negative are found. In overdoped Ba-K-Bi-O, the thermopower is wholly negative from Tc up to the limit of measurement near room temperature, and the temperature coefficient of thermopower is also negative. All the thermopower versus temperature plots for BaK-Bi-O show a characteristic bend around 150 K, with a smaller temperature coefficient above 150 K. As Ni is substituted for Cu in the 214 cuprate series, there is a substantial increase in the thermopower for Sr concentration x=0.15, but no such increase is seen in the overdoped samples x∼0.5.