The absorption spectra of the A1 low-frequency exciton band in quateary compound Rb0.5Cs0.5Ag4I5 thin tilms have been measured by an UV-2100 spectrophotometer at the temperature (T) range from 78 to 289K.The measurements at intermediate temperatures are performed within an interval of photon energies from 3.44 to 3.70 e V. The spectra have been computer-processed to separate the A1 band from the interband absorption edges,and to determine their parameters: the spectral peak position (Em), the half-width (G) of the exciton band, and the Gaussian shape ratio in the spectrum. The results show that at T ＜ 107K, dEm/dT = -6.34 × 10-4 eV.K-1,dG/dT = 3.04 × 10-4 eV.K-1; at T ＞ 107K, dEm/dT = -3.01 × 10-4 eV.K-1, dG/dT = 5.47 × 10-4 eV.K-1.It is emphasized that the phase transition γ→β in this material is carried out at a temperature of 107K.