Abstract A new sequential peptide, poly(leucyl-leucyl-aspartic acid) has been synthesized and the conformational properties in water have been examined by spectroscopic methods at different degrees of neutralization of the β-carboxyls of the aspartate residues and by potentiometric measurements. The experimental data are in agreement with the presence of α-helical segments stabilized by strong hydrophobic interactions. The calculation of the thermodynamic functions, ΔG 0, ΔH 0, ΔS 0 related to the transition uncharged random coil to uncharged α-helical state, gives at 25°C the following values: ΔG 0=−1575 cal/mol; ΔH 0=6173cal/mol; ΔS 0=26·0cal/deg mol. These values are expressed in cal/mol of repeating unit. The temperature dependence of the ellipticity and the function pH—log[ α (1−α) ] versus α, allows us to analyse the conformational stability of poly(leucyl-leucyl-aspartic acid).