The objective has been to establish if those ions which are known to change the stability of the struct,ure of proteins, have any influence on the properties of ionizable polypeptides. Potentiometric titrations and complementary optical rotation data are presented for aqueous solutions of poly-L-lysine (PLL) in the presence of KSCN, KC1, and KF, and for poly(L-glutamic acid) (PLGA) in the presence of KSCN, KCl, and LiCI. The following measured quantities which are affected by salt concentration were obtained: intrinsic pK (pKo), slope of pK,,, versus degree of ionization (a) curves, the degree of ionization a t which the helix to coil transition occurs, and the free energy of this transition for the uncharged molecule (AGh",l). The effects of nonspecific salts (KCl arid LiCl for PLL and KSCN and KC1 for PLGA) are small, and about as expected from general e1ectrostat)ic considerations. In line with the observations made with isoelectric and catioriic collagen, specific effects were noted with KSCN-PLL and with LiC1-PLGA. 111 the presence of KSCN, the poly-L-lysine helix becomes stabilized a t much lower degree of ionization than in the presence of KCl, and the slope of the pK,,, versus a plots is greatly reduced. However, AG&l (for the uncharged molecule) is not affected, attd pKo is orily slightly higher. We interpret these data in terms of binding of SCNprimarily to the side-chain aniiiio groups (both to It-NH3+ and to It-NH,). Poly(L-glutamic acid) in LiCl solution has its transit,ion a t the same a value as in KCl solution. However, both the slopes of the pK,,, versus a plots and the absolute values of AGE,, are lower than in KCl solution. We interpret these results ill terms of biiidiiig of Li+ to side chaiiis as well as to the pept,ide bond.