This in vitro study evaluated the pH value of active and arrested caries lesions in deep dentinal caries using a pH-imaging microscope (SCHEM-100, HORIBA Ltd, Kyoto, Japan). Buccal-lingual cut sections of extracted human teeth that had either active or arrested dentinal caries lesions were placed on the pH-imaging sensor of the microscope. The pH values were compared statistically by one-way ANOVA and Fisher's PLSD test (p<0.05). In addition, both types of caries lesions were compared with a caries detector solution for this pH-imaging characterization. For both active and arrested lesions in dentin, the lowest pH values in the caries lesions (range from 5.3 to 6.6) were lower than that of intact dentin (range from 6.8 to 7.4). There were statistical differences between the lowest pH value within the active lesion (range from 5.3 to 5.8) and the arrested lesion (range from 6.3 to 6.6) (p<0.05). Although the arrested lesion was unstainable and impermeable to the dye, there was a close relationship between dye staining and pH-imaging characterization within the active lesion from visual inspection.