We present an experimental study of structure formation in a polypeptide hetero-arm star block copolymer solution, obtained by swelling thin films in chloroformsolvent vapor to variable poly(γ-benzyl-L-glutamate) (PBLGlu) concentrations (cp). Direct observation by optical microscopy allowed us to follow in real time nucleation and growth of ordered three-dimensional structures of ellipsoidal shape. At low cp, growth stopped when cp decreased below the solubility limit (ccritical) but additional structures were formed when cp was rapidly increased to a higher value. Although water is not a solvent for this polymer, we demonstrate that water, even in trace amounts, is nonetheless considerably affecting solubility and consequently the process of structure formation. We have varied systematically the amount of water present in the environment. ccritical changed from about 0.53 (dry, i.e. desiccated surrounding vapour phase) viaccritical ≈ 0.16–0.25 for 30–50% humidity of the vapour phase to ccritical ≈ 0.03 for a vapour phase at 100% humidity. We attribute this change in solubility to complexation of water molecules with PBLGlu α-helix, which increases the interfacial tension between the polymer and the solvent. We have tested our hypothesis by replacing water with other non-solvents for the polymer. Only protic non-solvents changed the solubility of PBLGlu in chloroform.