Thin crystalline solid films of KBr1-xIx:Tl+ were prepared from a melt by a cell method and impurity absorption spectra were investigated at 11 K for the whole composition range. In the middle composition range no spectral traces of phase separation were observed, indicating that the films were in a state equivalent to that of a supersaturated solid solution. The A band of KBr:Tl+ and that of KI:Tl+ were persistent with each other and two additional bands attributable to the A bands of the mixed crystals were apparent in the range of 0.01 less than or equal to x less than or equal to 0.5. According to the results of Gaussian fitting of the bands, it was evident that two of them have threshold compositions with increasing I- ion composition x. In addition, the I- ion concentration around the impurities was found to be greater than the I- ion composition of the host materials. These findings are discussed in terms of a model, according to which the bands are associated with various I- ion configurations around the impurities.