Azobenzene moiety-containing methylcellulose (AB-MC) was prepared, the changes of the cloud point of its aqueous solutions as a thermotropic sol-gel transformation behaviour were investigated under irradiation with UV light. AB-MC's with degrees of substitution by the azobenzene moiety DSAB > 2,5 . 10(-2) were insoluble in water. The azobenzene moiety in the AB-MC was confirmed to be reversibly isomerized from the trans- to the cis-form under irradiation with UV light of wavelengths 390 > lambda > 310 nm and from the cis- to the trans-form under irradiation with visible light of lambda > 400 nm. The cloud point value of the aqueous solution of AB-MC increased under irradiation with UV light of 390 > lambda > 310 nm for the AB-MC's with a DSAB value in a certain range and the extent of the increase in the cloud point value was higher at higher concentrations of AB-MC.