Hydrodynamic and electro-optical properties have been investigated for polybutylisocyanate (PBIC) molecules in mixtures of tetrachloromethane and pentaphtorophenol (PFPh). It was shown that variation of PFPh content (up to 20 per cent) leads to a reversible 5-fold change in the equilibrium rigidity of PBIC macromolecules. Coiling of PBIC chains with increasing PFPh concentration in solution arises from the effect of the aromatic component of the solvent on the conjugation in the main chain of the polymer. It was established that the conformational transformations in PBIC chains cannot be likened to the coil-helix transition in synthetics polypeptides.