Time-dependent material functions of engineering plastics within the exploitation range of temperatures extend over several decades of time. For this reason material characterization is carried out at different temperatures and/or pressures within a certain experimental window. Using the time-temperature and/or time-pressure superposition principle, these response function segments can be shifted along the logarithmic time-scale to obtain a master curve at selected reference conditions. This shifting is commonly performed manually ("by hand") and requires some experience. Unfortunately, manual shifting is not based on a commonly agreed mathematical procedure which would, for a given set of experimental data, yield always exactly the same master curve, independent of person who executes the shifting process. Thus, starting from the same set of experimental data two different researchers could, and very likely will, construct two different master curves. In this paper, we propose a closed form mathematical methodology (CFS) which completely removes ambiguity related to the manual shifting procedures. This paper presents the derivation of the shifting algorithm and its validation using several simulated-and real-experimental data. It has been shown that error caused by shifting performed with CFS is at least 10-50 times smaller then the underlying experimental error. (C) 2011 The Society of Rheology. [DOI: 10.1122/1.3503529]