The optical absorption behaviour of thin films of ZnIn2Se4 formed by a vacuum deposition process was studied. The optical constants (the refractive index n, extinction coefficient k and dielectric constants epsilon’ and epsilon ", are estimated for ZnIn2Se4 thin films as well as the effect of heat treatments in the wavelength range 300-2500 nm. X-ray diffraction and electron microscopy were used to obtain an insight into the structural information. ZnIn2Se4 is a layer semiconductor of tetrahedral crystal structure. From the reflection and transmission data, the absorption coefficient was computed for amorphous and crystalline films. Analysis of the absorption coefficient data revealed the existence of allowed direct and indirect transitions with optical energy gaps E(d)(opt) = 3.38 eV and E(i)(opt) = 2.22 eV at 300 K. These values were found to decrease with increasing annealing temperature.