Electrospray ionization-mass spectrometry (ESI-MS) was employed to study methanol-induced conformational changes in adrenocorticotrophic hormone (ACTH). ACTH, a 39–residue peptide, is a member of the proopiomelanocortin family of peptides. Charge-state distribution (CSD) and hydrogen–deuterium (H/D) exchange were used to monitor the conformational changes as a function of methanol concentration. The latter experiments were conducted via time-resolved ESI-MS in a continuous-flow apparatus. The CSD and the H/D exchange experimental data both reveal that ACTH exists, presumably in a random coil open structure in aqueous media, but assumes a more compact helical conformation with increased concentration of methanol. The H/D exchange experiments also reveal that 79% of ACTH is present as α-helix in mixed water-methanol solvent media.