The microscopic behavior of viscous materials under compaction was studied, with focus on the evolution of the pore structure with increasing pressure, at different strain rates. Granular polyethylene glycol (PEG) and high density polyethylene (HDPE) were compacted with a compaction simulator up to different pressures, at two different strain rates. Compaction curves were constructed, and diametral strength tests were performed on the tablets. Scanning electron microscopy (SEM) was used to characterize the microstructure. PEG exhibited a visco-plastic behavior, as opposed to HDPE, which behaved visco-elastically. Observation of the pore structure revealed that PEG fractured and developed rate-dependent permanent deformation, resulting in good bonding and strong tablets. On the other hand, HDPE tablets contained large pores, even at high pressures, due to the considerable amount of springback after ejection, and their diametral strength was low. For PEG, the out-of-die microstructure was strongly dependent on the strain rate, while the HDPE structure was almost independent of it. In both cases, the diametral strength and the in-die density were dependent on the strain rate. However, the effect of strain rate on diametral strength was considerably more evident for PEG than for HDPE, due to their different nature and to the resulting pore structure.