The concept of temperature shift factor (aT) as defined by Doolittle, relating the free volume of a viscoelastic material at the current and reference states is briefly examined together with the resultant William-Landel-Ferry equation. This paper highlights the fact that change in free volume arise not only from temperature variations but can also result from the absorption of solvents and thus a generalized Doolittle relation may also be applied to a solvent concentration shift factor (ac). To validate this concept, a small scale laboratory investigation was carried out by blending 40/60 penetration grade bitumen with various proportions of one type of cooking oil and conducting dynamic shear rheometer frequency sweeps at a range of temperatures. By applying time-concentration superposition to each flux content, it was possible to shift horizontally (ac) each set of complex modulus data measured at each test temperature, so that all sets superimpose onto the master curve of the base bitumen at a preselected reference temperature. A direct relationship between conventional time-temperature shift and the proposed time-concentration shift factors was thus demonstrated using a sample of penetration grade bitumen and one type of vegetable oil. Further experimentation with other bitumen-flux combinations is necessary prior to recommending general adoption of the proposed tool.