Secondary drying involves removal of water which did not freeze. This report emphasizes the phenomenological description of the effects of temperature and chamber pressure on the kinetics of drying. A crystalline solute (mannitol) and two amorphous solutes (moxalactam di-sodium and povidone) were selected for study. Drying kinetics were determined gravimetrically using a vacuum microbalance and by Karl Fischer assay of vials sealed at selected times during secondary drying experiments conducted in a laboratory scale freeze dryer. The main observations may be summarized as follows: (1) the water content decreases rapidly during the first few hours of drying and then appears to approach a plateau level of residual water which far exceeds the equilibrium water content calculated from the desorption isotherm data and the measured partial pressure of water in the drying chamber; (2) this plateau level of water sharply decreases as the drying temperature is increased; (3) the drying rate increases as the product specific surface area increases; and (4) variations in chamber pressure (0–0.2 mmHg) and dried product thickness have little or no effect on drying rate. We conclude that the rate-limiting mass transfer process for drying an amorphous solid is either evaporation at the solid/vapor interface or diffusion in the solid, probably the former. The ‘plateau level kinetics’ appears to be consistent with amorphous particle size heterogeneity superimposed on a simple model based on Fickian diffusion with rate controlling surface evaporation.