An ex-situ adsorption measurement method based on photoelectrochem. oxidn. principle has been was developed. The ex-situ adsorption method involves a washing step for the titania surface after the adsorption step. In this work, an in-situ photoelectrochem. detn. of adsorption of org. compds. and water on titania is developed. By eliminating the washing step after adsorption, this proposed in-situ method is capable to study the adsorption behavior in an extremely small time scale (e.g., < 1s). To our knowledge, this has not been achieved in the literature. The proposed method can directly quantify the amt. of adsorbates at a titania surface in a sensitive, reproducible and accurate manner. The adsorption of phthalic acid was studied as a test case. The relationship between the adsorption amt. at the titania surface and the concn. of phthalic acid in the soln. was obtained.