The fully Cn-methylated homo-peptide Ac-[L-(alpha Me)Val](7)-NHi Pr is completely 3(10)-helical when its crystals are grown from a methanol solution, whereas it is alpha-helical when crystallized from HFIP (1,1,1,3,3,3-hexafluoropropan-2-ol), thus providing an example of a solvent-driven alpha/3(10)-helix dimorphism in the crystal state. The interactions of cocrystallized HFIP molecules with the peptide in the alpha-helical structure are reported.