We describe the first determination of thiol compounds with gold nanocomposites composed of gold nanoparticles and thermoresponsive copolymers having polyamino groups. The gold nanocomposites, which are used as a chromatic sensor, reveal chromatic change from blue to red with thermal stimuli, heating followed by cooling the solution. The blue-to-red chromatic change results from disassembly of the gold nanocomposites, which arises from shrinkage of the thermoresponsive copolymers bound to the gold nanoparticle surfaces due to the phase transition induced by,thermal stimuli. The disassembly is inhibited by addition of thiol compounds through displacement of the adhered thermoresponsive copolymers. The detached copolymers no longer influence morphological change of the gold nanocomposites. Corresponding with increase of concentration of the thiol compounds, a solution of the gold nanocomposites after the thermal stimuli shows chromatic change, which was quantified with the a* value in L*a*b* chromatic coordinates. A linear relationship between the a* value and concentration of cysteine, examined as a bio-important thiol, is obtained below 7 x 10(-6) mol dm(-3), estimating a detection limit defined as 3 sigma of the blank to be 2.8 x 10(-7) mol dm(-3). The chromatic sensor of the gold nanocomposites is applied to the determination of cysteine in commercial supplements containing ascorbic acid, which seriously interferes with redox-based determination of cysteine. Analytical results obtained with the chromatic sensor are identical to those obtained with HPLC.