CsGaS2-mC64 was obtained by reaction of CsN3 with stoichiometric amounts of Ga2S3 and S at elevated temperatures. The crystal structure of the air- and moisture stable compound was determined from single-crystal X-ray diffraction data. The colourless solid crystallizes in the monoclinic space group C2/c (no. 15) with the lattice parameters a = 10.5718(6) angstrom, b = 10.5708(6) angstrom, c = 16.0847(8) angstrom, ss = 99.445(4)degrees, V = 1773.1(2) angstrom(3), and Z = 16. The compound crystallizes in the TlGaSe2 structure type and features anionic layers (2)(infinity)[Ga4S84-] consisting of corner-sharing Ga4S10 supertetrahedra. At temperatures above 600 degrees C an irreversible phase-transition to CsGaS2-mC16 occurs. The phase-transition kinetics were studied using in situ high-temperature X-ray powder diffraction techniques. This transition can only be reversed by using high pressures (> 5 GPa at 500 degrees C). The compound was further characterized using Raman- and diffuse reflectance spectroscopy. Chemical bonding was analysed by DFT calculations.