Rheological phase transition behavior of MWCNT/PS composites with various MWCNT contents was characterized by using a frequency sweep experiment with an oscillatory rheometer at 210 degrees C. At 210 degrees C, it revealed that the MWCNT content at the rheological phase transition of MWCNT/PS composites with different contents of dispersing agent was measured to be 3.6 wt% for MWCNT/PS composite without dispersing agent, 3.0 wt% for the composite with 10 wt% of dispersing agent, and 3.2 wt% for the composite with 20 wt% of dispersing agent. This rheological transition could be related to a physical gel formation of the composites, providing an alternative way to determine the degree of dispersion of MWCNTs in the composites. (C) 2007 Elsevier B.V. All rights reserved.