Measured at 2 degrees C in water, NMR chemical shifts of C-13=O labeled central alanine residues of peptides W-LyS(5)-'L-3-Ala(n)-'L3-Lys(5)NH(2), n = 9, 11, 13, 15, 19 and W-Lys(5)-L-t(3)-a-Ala(n)-A-Inp-(t)L2-Lys(5)NH(2) (a = D-Ala; L-t = tert-leucine; Inp = 4-carboxypiperidine) are used to assign j(L)(t) and C-L(t), the N- and C-terminal tL capping parameters and length-dependent values for W-Ala(n), the alanine helical propensity for Ala(n) peptides. These parameters allow Lifson-Roig characterization of the stabilities of Ala, helices in water. To facilitate chemical shift characterization, different C-13/C-12 ratios are incorporated into specific Ala sites to code up to six residue sites per peptide. Large left/right chemical shift anisotropies are intrinsic to helical polyalanines, and a correcting L-R-based model is introduced. Capping parameters j(L)(t) = C-L(t) lie in the range of 0.3 to 0.5; the L-t residues are thus moderately helix-destabilizing. For helical conformations of lengths shorter than eight residues, assigned values for W-Ala approach 1.0 but increase monotonically with length to a value of 1.59 for W-Ala(19).