The aim of this work was to determine the three-dimensional thermorheological behavior of isotactic polypropylene (i-PP) in the region of its glass transition temperature (T-g) by a master curve. The i-PP is a widespread polymer with a T-g similar to 0degreesC. Dynamic mechanical analysis (DMA) at varying frequencies and temperatures and bulk tests at varying temperatures and times are carried out to obtain the relaxation spectra. Traditionally, the combination of time and temperature is done for thermorheological simple material by the creation of a master curve based on the Arrhenius or William-Landel-Ferry (WLF) equation. This investigation shows that these equations do not fit the behavior across the glass transition of i-PP. Instead, a new arc tangent function is derived. Additionally, it can be shown that the shifting factors differ from shear to bulk load. Therefore, the mode of mechanical stress seems to have an influence on the thermorheological behavior. (C) 2004 Wiley Periodicals, Inc.