A simple additive model for the isotropic, dipole polarizabilities (alpha) of molecules is considered. It is successfully tested on ab initio MP2 values of several small molecules. Then it is used to predict values of alpha for the 20 proteinogenic amino acids. The results are in good agreement with previous ab initio calculations and, through cancellation of errors arising from the neglect of some small effects, also with experiment. (C) 2009 Elsevier B.V. All rights reserved.