Abstract: An acid sensitive colour-changing material,composed of halogenated copolymer and acid indicator,has been developed. In order to improve its image quality,the photochemical reaction involved was studied. It was shown that the VDCl₂-MA undergoes a simple process of photolysis with the release of HCl,based on -the spectral characteristics of VDCl₂-MA copolymer before and after exposure to UV irradiation and the ESR and ESCA measurements upon exposure to UV. The experimental results show that upon UV irradiation the tran-form DMY is converted into cis-form,and the cis-form is unstable and turns back easily into the trans-form when the irradiation is ceased. The rate of the reverse reaction follows the first order kinetics and the rate constant is 1.1×10^-2 sec^-1,measured by flash photolysis. The image recorded by the VDCl₂-MA and DMY photographic materials undergoes some fading in storage. By the addition of phenols,the image stability was improved.