The solvent-dependent c.d. behaviour of terminal N-thiobenzoyl derivatives of twenty peptides has been determined. The terminal N-thiobenzoyl derivative of a peptide carrying an N-terminal L-α-amino-acid residue shows positive n→π* c.d. in aqueous solution; the derivative of a peptide carrying an N-terminal L-α-imino-acid residue (L-proline) shows negative n→π* c.d. in water. The penultimate amino-acid residue in these derivatives also contributes to their c.d., directly by perturbation of the n→π* thiobenzamide transition and indirectly by influencing conformational equilibria involving the substituted N-terminal amino-acid residue.