The UV irradition of K [RCF = CFBF3] [R = C4F9 (trans). C2F5 (cis)- C6F13 (cis), Cl (cis/trans 1 : 1)] in acetone led to cis/trans-isomerization with a final cis/trans composition 7:3. In the case of R = C4H9 (trans) or C3F7O (cis/trans 25 : 75) the photoisomerization was accompanied by a partial decomposition.