We consider the possibility that significant effects due to transient creep might conceivably contaminate the measurement of mantle viscosity which one obtains from postglacial rebound data. A time domain mapping is employed which allows us to consider Maxwell and standard linear solids in which the viscosity is time dependent. This technique is used to investigate the impact which either of two recently proposed transient rheologies would have upon inferences of viscosity from isostatic adjustment data. The free decay regime relaxation data from a site near the center of the Laurentide depression (Richmond Gulf) are invoked to show that transient effects, if they exist at all, are most probably weak.