We have measured the thermopower and resistivity of bulk superconducting samples of HgBa2CuO4+y (Hg-1201). Various annealing treatments under flowing oxygen and argon were carried out in order to change the oxygen content and thereby the charge-carrier density. As the oxygen content is increased, both the thermopower and resistivity decrease, and the thermopower becomes negative with a negative linear slope. The overall pattern of the thermopower and Tc variation is similar to that found recently for the Tl-1201 superconductor series. © 1994.