The homolytic bond dissociation energies (BDEs) of the acidic C-H bonds in the following families wherein R is changed along the series Me, Et, i-Pr, and t-Bu have been estimated : (a) 9-alkylfluorenes (9-RF1H), (b) 9-RCH(2)FlH, (c) 9-RSFlH, (d) 9-RSO(2)FlH, (e) (RSO(2))(2)CH2, (f) RSCH(2)CN, (g) 9-ROFlH, (h) PhCOCH(2)XR (X = O or S), (i) RSCH(2)CO(2)Et, (j) 9-R(2)NFlH, and (k) 9-piperidinylfluorenes. For the 9-RFlH family there is a progressive decrease in acidities from 9-MeFlH to 9-t-BuFlH caused by steric hindrance to solvation, and the BDEs show a progressive increase along the series. On the other hand, for the 9-RCH(2)FlH family there is a progressive increase in acidities from 9-MeCH(2)FlH to 9-t-BuCH(2)FlH and the BDEs remain essentially constant. The acidity increases for this family and for the families c-k are believed to be caused by progressive increases in anion stabilizing polarizability effects of the R group in RCH(2), RS, RSO(2), RO, and R(2)N with increasing alkyl size. The pattern of behavior observed wherein the acidities increase but BDEs remain constant is dictated by the ability of polarizability effects to stabilize anions, but not analogous radicals. Similar behavior, but with much larger polarizability stabilizing effects, has been observed previously for the Ph(3)P(+) group in Ph(3)P(+)CH(-)G ylides.