Thin crystals of Na1−xKxI:Tl+solid solutions with thicknesses of 0.1 ∼ 10 μm have been produced and their phase separation has been observed in both regions of the exciton band and Tl+impurity band. An intermediate band of Tl+was recognized in the absorption spectrum of Na0.5K0.5I:Tl+. A precursor phenomenon was found as a temporal change of the intensity ratio of two Tl+emission bands.