A terahertz chemical microscope (TCM) has been developed to visualize the chemical concentration of solutions in fluidic chips. Chemical potential-terahertz transducers were newly developed and applied to the bottom surfaces of the flow channels. When femtosecond laser pulses hit the transducers, terahertz waves are radiated from them. Since the amplitude of terahertz waves can be related to the chemical potential on the surface of the device, the distribution of chemical reactions on the surface can be visualized by scanning the laser on the device. Here, we report the demonstration of mapping of pH values using TCM with a spatial resolution of less than 1 mm. The pH distribution of the flow channels filled with 0.01 M NaOH and HC1 was successfully visualized. In addition, the neutralization process was monitored. © 2009 The Institute of Electrical Engineers of Japan.