The helicity reporting parameters t/c and [theta](222) have been measured at 2, 25, and 60 degreesC in water for the solubilized polyalanine series Ac-Hel-A(n)-(t)Llnp(2)K(4)W-NH2 of length 4 less than or equal to n less than or equal to 14 that bears the helix-initiating and monitoring N-cap Ac-Hel and the spaced solubilizer (t)Llnp(2)K(4)W-NH2 as a C-cap. Correlation of t/c with length shows that the helical propensity for n less than or equal to 6 is ca. 1.0, consistent with our early reports, but that a dramatic increase in temperature dependence and helical propensity occurs for n greater than or equal to 8. A model based on hydrogen-bonding cooperativity is proposed to explain this finding, and both t/c and [theta](222) are modeled successfully by length-dependent alanine propensities at 2 degreesC of 1.03 for n = 6, 1.15, for 7 less than or equal to n less than or equal to 9 and 1.26 for n greater than or equal to 10. The implications of these results for the energetics of helix formation by alanine-rich peptide sequences are discussed.