Electrical conductivities were measured for methane, benzene, and polybutene shock compressed to pressures in the range 20 to 60 GPa (600 kbar) and temperatures in the range 2000 to 4000 K achieved with a two-stage light-gas gun. The data for methane and benzene are interpreted simply in terms of chemical decomposition into diamondlike, defected C nanoparticles and fluid H-2 and their relative abundances (C:H-2), 1:2 for methane and 2:1 for benzene. The measured conductivities suggest that conduction flows predominately through the majority species, H-2 for methane and C for benzene. These data also suggest that methane is in a range of shock pressures in which dissociation increases continuously from a system which is mostly methane to one which has a substantial concentration of H-2. Thermal activation of benzene conductivities at 20-40 GPa is probably caused by thermal activation of nucleation, growth, and connectivity of diamondlike, defected C nanoparticles. At 40 GPa the concentration of these C nanoparticles reaches a critical density, such that further increase in density does not have a significant affect on the cross-sectional area of conduction and, thus, conductivity saturates. The electrical conductivity of polybutene (1:1) is very low. While the mechanism is unknown, one possibility is that the electronic bandgap of whatever species are present is large compared to the temperature. Electrical conductivity measurements are proposed as a way to determine the melting curve of diamondlike C nanoparticles at 100 GPa pressures.