To investigate the behavior of the As4S4 molecule within a crystal-chemical environment differing from realgar, α-As4S4, and its high-temperature polymorph, β-As4S4, the effects of the light exposure on the structure of the HgI2·As4S4 adduct have been studied. The structure of this compound consists of a packing of nearly linear HgI2 molecules and As4S4 cage-molecules. A crystal [V = 1295.9(4) Å3] was exposed to filtered polychromatic light (550 nm long-wavelength pass filter). A marked increase of the unit-cell volume as a function of the exposure time was observed up to V = 1338.9 Å3 at 3060 min of exposure. Structure refinements indicated that the increase of the unit-cell volume is to ascribe to the formation of an increasing fraction (up to 59%) of pararealgar replacing the realgar-type molecule. After this point, further light-exposure did not cause any further increase of the lattice parameters. On the contrary, a slow, continuous decrease of the unit-cell volume down to 1292.9 Å3 occurred by keeping the crystal in the dark (39 days). The “reverse” evolution of the unit-cell parameters in the dark almost overlap that observed during the light-induced process and the structural model obtained after the “dark stage” was found to be identical to that of the unaltered crystal, although the diffraction quality was lower and powder-like diffraction rings were observed together with single-crystal reflections. Apart from few peaks belonging to the original unaltered HgI2·As4S4 adduct, most of the collected peaks can be assigned to β-As4S4. This feature could indicate decomposition into micro-crystalline β-As4S4 and HgI2; no diffraction effect ascribable to any HgI2 phase, however, was observed. Micro-Raman spectra were collected on crystals exposed to the above-mentioned wavelength light for increasing times (up to 3000 min). The peak at 274(±1) cm−1 whose intensity increases as a function of the exposure time confirms the transition from a realgar- to a pararealgar-type molecule in the HgI2·As4S4 adduct.