We report a conformational change observed in poly-n-butyl isocyanate (PBIC) dissolved in carbon tetrachloride (CCl4) with temperature. This change was detected by measuring the reduced viscosity of the polymer solution with a Zimm-Crothers type viscometer of very low shear rate; we found a reversible jump in reduced viscosity whose amplitude depends on concentration, but its position is only molecular weight dependent. This jump goes from lower to higher values of reduced viscosity when we increase the temperature, corresponding to an increase in the hydrodynamic volume of the chain. The largest size of the jump we found was about 3.5% of the viscosity value, and the jump decreased when the concentration approached the overlap concentrationc*.