Chalcogenide thin films are currently used in Phase Change Rewritable Optical Recording media such as CD-RW and PD. These media have storage capacities 650 Mbytes, and can be rewritten over 1000 and over 100,000 times respectively. The second generation of Phase Change Rewritable media, using the DVD format, will extend that capacity to 4.7 GBytes/side. At the heart of this technology is the Chalcogenide layer, which undergoes a reversible change between amorphous and crystalline structures upon absorption of appropriate laser energy. The accompanying change in the optical constants [n,k] of the chalcogenide layer results in large reflectivity differences of the multi-layer media. Design of Phase Change optical media requires precise determination of [n,k] for both structures of the alloy over a wide spectral range. We have accomplished this task from measurements of [R,T] using the [air/film/substrate/air] configuration for films < 50 nm thick. The film's [n,k] are obtained by direct numerical inversion of the appropriate [a/f/s/a] expressions relating [n,k] to the measured [R,T]. Since R and T are highly non-linear in [n,k], calculating [n,k] provides multiple solutions at each wave length including the physical solution and also other mathematically relevant solutions. The other solutions can be used to determine the film's average thickness to an accuracy of ~ 1% or better. Using this technique we show that when as-deposited amorphous Chalcogenide films crystallize upon annealing they typically undergo a reduction in thickness of ~ 10%. We have developed software which can calculate [n,k] at 1800 wavelength points in less than 30 seconds. Attaining the correct film thickness requires only a few iterations. The calculation methodology does not include any assumption regarding the dispersion relation of the unknown film. This is critically important when multi-layered media structures are developed using new alloys.