Dynamic mechanical data obtained in the tensile mode with cellophane, polyamide 6, PEG, PVAc and PUR foam (compression) subjected to stepwise humidity changes between 5% and 85% relative humidity are reported. With the exception of PVAc, the loss tangent, tan delta, shows a transient peak every time the relative humidity of the surrounding atmosphere is changed. The strength of the tan delta transients increases when the measuring frequency is lowered. The tan delta transients slowly disappear in response to reduction of the rate of humidity change. High amplitude and prestress with static load reduce the height of the transient peaks. Long-term testing results in relatively small changes in the shape of the loss tangent versus time data. For PA6 a narrowing of the loss tangent peaks with diminishing specimen thickness was observed.