Radiophotoluminescence (RPL) is an emission phenomenon resulting from the light excitation of RPL centers formed by hole and electron transfer upon irradiation with γ-or X-rays. The RPL intensity is proportional to the absorption dose, and therefore, RPL is applied in dosimetry. Many materials showing RPL have been investigated. Amongst them, Ag-doped alkali halides are the earliest known compounds with RPL, but the mechanism underlying RPL center formation has not been well understood. In this study, we investigated the RPL center formation in Ag-doped alkali halides by combining fluorescence and electron spin resonance (ESR) spectroscopy. An emission peak attributed to Ag0 and an ESR signal attributed to the holes trapped at the Cl- sites were observed in the spectrum of the irradiated Ag-doped NaCl sample. Therefore, we suggest that in Ag-doped NaCl, electrons are trapped at the Ag+ sites and form Ag0, while holes are trapped at the localization centers of Cl- and are expected to form VK centers. Conversely, an emission peak attributed to Ag0 was observed in the spectrum of the irradiated Ag-doped KCl sample. In addition, ESR signals related to Ag0 and Ag2+ were observed in the spectrum of the non-irradiated Ag-doped KCl sample. These results suggest that electrons and holes are trapped at the Ag+ sites to form Ag0 and Ag2+.