A simple group contribution scheme is presented to predict the glass transition temperature, Tg, of pure polymers and pure diluents, also called additives, once their chemical structures have been defined. This method considers that the glass transition of any polymer can be calculated from the contribution of the groups that make up the main and side chains of the repeating unit, while the Tg of any diluent can be estimated from the contribution values reported here as if the groups present in the diluent were part of the side chain of a polymer with no "main backbone". The 66 contribution values reported are intended for very small groups and may be used to predict the glass transition temperature of linear polymers in their most atactic forms, polymers having side-chain groups, or diluents based on the elements C, N, O, H, S, P, Si, and halogens. Phenyl and naphthalene rings are also included. Compared to Bicerano's method, the proposed method is as effective as his; however, it has the advantage of considering neither solubility parameter nor structural factors. In addition, it may be used to predict the Tg's of pure diluents.