A simple and reliable FT-IR method enabling almost complete qualitative and quantitative identification of the components of unsaturated polyester resins for glass-reinforced laminates is presented. This method is based on percentage transmission ratios of characteristic bands in a single IR spectrum. Coupled with simple chemical tests, such as resin acid number and, when needed, UV-VIS determination of Co (II) curing accelerators, this FT-IR method allows the qualitative and quantitative determination of the various reacted and unreacted components of the resin. Some critical discussion of the advantages of such a method over some conventional tests is presented.