Carbon black/polymer composites can be used in a variety of industrial applications, where a high conductivity is an important physical parameter. The relationship between the positive temperature coefficient (PTC) effect and the room temperature volume resistivity has been investigated. These are two of the more important operating parameters. Resistivity-temperature measurements were performed on a radiation crosslinked high structure (H) carbon black mixed at different concentrations in HDPE. The results obtained for composites with a range of carbon black properties showed that there was a linear relationship between log (R-2/R-1) and log R-1, where R-2 = maximum resistivity and R-1 = room temperature volume resistivity. The relationship can be taken as a reference line for the assessment of the relative effect that modifying the carbon black structure and changing the mixing variables on the relative magnitude of the PTC effect and the room temperature volume resistivity. The high structure (H) carbon black structural characteristics were modified by both gas phase (carbon dioxide at 900 degrees C) and liquid phase (nitric acid) treatments. The former treatment procedure resulted in a decrease, while the latter an increase in the volume resistivities of the final composites. These results are discussed in terms of the effect of surface functionality and carbon structural characteristics.