Thin Rbl-xCsxI mixed crystals have-been grown from the melt in a very small gap formed in between a pair of quartz plates and reflection spectra were measured in the uv-light region at low temperature. Continuous spectral changes of the exciton bands with the;composition x suggest strongly the formation of extended solid solution both in the RbI-rich (0.0 less than or equal to x less than or equal to 0.4) and CsI-rich (0.7 less than or equal to x less than or equal to 1.0) regions. The formation was confirmed by x-ray diffraction, and the compositional change of the-lattice constant was determined. These results are in contrast to the eutectic:type phase relation with only a limited solid solubility observed in the built state of the RbI-CsI binary system.