Disclosed is a process for converting an iodine-free organotin (IV) compound resulting from the decomposition of an organotin (IV) iodide during the catalytic isomerization of η, δ-epoxyalkenes to 2,5-dihydrofurans to the corresponding, catalytically active organotin (IV) iodide by intimately contacting a catalyst mixture comprising (i) an organotin (IV) iodide, (ii) an iodine-free organotin (IV) compound resulting from the decomposition of the organotin (IV) iodide of component (i), and (iii) a solvent selected from hydrocarbons and chlorocarbons with aqueous hydrogen iodide.