Using the SG-III prototype laser at China Academy of Engineering Physics, Mianyang, we irradiated polystyrene (CH) samples with a thermal radiation drive, reaching conditions on the principal Hugoniot up to P ≈ 1 TPa (10 Mbar), and away from the Hugoniot up to P ≈ 300 GPa (3 Mbar). The response of each sample was measured with a velocity interferometry diagnostic to determine the material and shock velocity, and hence the conditions reached, and the reflectivity of the sample, from which changes in the conductivity can be inferred. By applying the self-impedance mismatch technique with the measured velocities, the pressure and density of thermodynamic points away from the principal Hugoniot were determined. Our results show an unexpectedly large reflectivity at the highest shock pressures, while the off-Hugoniot points agree with previous work suggesting that shock-compressed CH conductivity is primarily temperature-dependent.