Reproducible p-type Ag-S co-doped ZnO (ASZO) films were grown on glass substrate by a modified hydrothermal method, where reaction solution is separated from alkali solution and O-2 is removed from autoclave to avoid deoxidation of Ag+ and oxidation of S-2 in the solution. It is found that the Ag substitutes for the Zn in monovalent state (Ag-Zn(+)) and the S for the O in bivalent state (S-O(2-)) in the ASZO. Both Ag-Zn(+) and S-O(2-) have two doping states, each Ag-Zn(+) is surrounded by 4O(2-) to form Ag-Zn(+) acceptor and each S-O(2-) is surrounded by 4(Zn)(2+), while Ag-Zn(+) bounds with nS(O)(2-) to form Ag-Zn(+)-nS(O)(2-) (n <= 4) complex acceptor. It is demonstrated that Ag doping can suppress the formation of interstitial Zn and O vacancies in the ASZO and S doping can enhance solubility of Ag in the ASZO. Hall measurement indicates that the p-type ASZO film has a hole density as high as 10(18) cm(-3), which is much higher than that of ASZO prepared by magnetron sputtering. The mechanism of the formation of the p-type ASZO with high hole density is suggested in the present paper. (C) 2016 Elsevier B.V. All rights reserved.