Practically all UV absorption spectroscopy is done in solution, and examples of its use in examination of neat (one component) solid materials are relatively few. In the IR on the other hand, the examination of thin films and microtomed sections of elastomers and plastics is a normal procedure. The reasons for this are fairly obvious: scatter by optically imperfect films is much more serious in the UV, many polymers give no specific UV absorption, and those that do absorb often give such intense absorption that impracticably thin films would be required. The main value of UV is in examination of trace amounts of UV-absorbing materials in those polymers which are more or less transparent, the best example being the study of crosslinking of elastomers. This work cannot be done in solution because the cross-linked polymers are insoluble in all solvents. A great deal of the work on the chemistry of vulcanization has therefore been done on model compounds of low molecular weight, most notably at the Natural Rubber Producers' Research Association. Although the use of model olefins has yielded valuable information, investigations on polymers are required because the different conditions, particularly in steric factors, may appreciably alter the balance of reactions.