It is shown that the glass transition temperature T-g of molecular (non-polymeric) glass formers correlates with molecular mass M as T-g(M) alpha M-alpha, alpha = 0.51 +/- 0.02. The subclasses of molecular glasses with homologous chemical structure but different M exhibit a similar universal correlation with significantly lower scatter. A possible explanation of T-g vs M correlation in molecular glasses is suggested. Comparing molecular glasses with polymers we found that in polymers T-g(M) dependence at small M (short chains) is similar to that in molecular glasses. At further increasing of the chain length the T-g(M) dependence in polymers begin to deviate from the universal T-g(M) correlation, of molecular glasses and eventually saturates at some polymer specific T-g infinity value. We conclude that at least a substantial part of T-g(M) dependence of low-M polymers is common with molecular glasses mechanism that does not require chain-like structure. In particular, the model of T-g(M) dependence in polymers based on additional free volume on chain ends is not fully adequate at small M. Our picture provides an alternative explanation that in polymers a mechanism is in action which leads to a saturation of the normal T-g(M) dependence common with molecular glasses. (C) 2013 Elsevier Ltd. All rights reserved.