An extreme and reversible growth of electrical resistance as a response of polyisoprene–high structure carbon nanoparticle composites (PHSCNC) to tensile or compressive strain found in earlier studies is examined with the purpose to adapt the material as a highly sensitive sensor of organic solvents. The change of the electrical resistance of PHSCNC studied during exposure to different organic solvent vapours is reported. The highest sensitivity is detected at concentrations of 9–11 mass parts of the nanoparticle filler corresponding to percolation region of the composite. The mechanism of vapour sensing is studied and the extremely high sensitivity (remarkable and rapid change of the resistance) is explained by tunnel current between adjacent carbon nanoparticles. The rate of the change of resistance is found to depend on the chemical composition of the molecules being absorbed by the polymer matrix.