Transient absorption spectra have been detected after flash photolysis of iodine in a variety of solvents. There is some correlation between the wavelength of maximum absorption and the ionization potential of the solvent and the spectra are attributed to the charge transfer complexes between iodine atoms and the solvent which were predicted in part III. In the case of methyl iodide as chaperon a weak transient has also been observed in the vapour phase. The decay of the complex follows a second-order law and rate constants have been measured in six solvents. Values for the extinction coefficient are derived by assuming that the reaction is diffusion controlled. Equilibrium constants of complex formation have been measured in ethyl iodide and ethyl bromide.