The photodegradation reaction rate of CHCl 3 in TiO 2 particulate suspension was improved significantly by HCl-treatment. The effect of HCl-treatment on the photocatalytic activity of TiO 2 was studied in a PEC cell by using Intensity-Modulated Photocurrent Spectroscopy (IMPS). The magnitude of photocurrent response and the characteristic frequencies of the upper and lower semicircles in the complex plane of IMPS response were analyzed, and the cathodic and anodic reaction processes of photogenerated holes and electrons were discussed. The increases in the cathodic and anodic photocurrent response and in the time constants of both cathodic and anodic reaction processes indicate that HCl-treatment leads to the improvement of the photocatalytic activity of TiO 2 and a change of the photocatalytic kinetic mechanism. The effect of HCl-treatment on the photocatalytic activity of TiO 2 was studied in a PEC cell by using Intensity-Modulated Photocurrent Spectroscopy (IMPS). The magnitude of photocurrent response and the characteristic frequencies of the upper and lower semicircles in the complex plane of IMPS response were analyzed, and the cathodic and anodic reaction processes of photogenerated holes and electrons were discussed. The increases in the cathodic and anodic photocurrent response and in the time constants of both cathodic and anodic reaction processes indicate that HCl-treatment leads to the improvement of the photocatalytic activity of TiO2 and a change of the photocatalytic kinetic mechanism.