The I*I*(0(9)(+)) --> I*I(B(0(u)(+))) emission spectra obtained in neat Ar, Kr, Xe, and in mixed Kr/Xe matrixes is reported. The emission from the strictly cage-bound initial state shifts linearly with Kr/Xe mixing fraction, without significant change in spectral width. The line shapes in mixed-hosts are understood in terms of the statistics of sites in the solid solutions. The pure-host line shapes are reproduced through Monte Carlo simulations using accurately known I-Rg potentials. The analysis yields a refined I*I* potential and provides a calibration curve to readout the cavity size of isolation from the observed emission peak shift.