The effect of time that processed cheese was held ex- posed to heat alone or with agitation, the level of rework cheese, and the type and level of emulsifying salt were evaluated by determining finess of emulsion based on scanning electron microscopy measurements, meltability, and rheological measurements using a Universal Testing Machine. Process cheese held in the cooker at 82°C for up to 4o minutes became less meltable and more firm and the emulsion became finer while that held at 82°C outside of the cooker without agitation was only slightly affected. Loss of meltability and increased firmness associated with pro-longed cooking of process cheese is associated with the state of the fat emulsion and not with the effect of heat on the proteins per se. The level of rework cheese had a great influence on the cheese rheological properties as well as on the microstructure of process cheese. There was a poor emulsion when no emulsifying salt was used regardless of the percent rework cheese added. The emulsion became finer as the per- cent rework cheese was increased if emulsifying salt was also present except after 20% rework cheese addition where the emulsion became coarse again. Process cheese lost meltability during storage at 4°C after 53 days when 1.0 or 0.0% emulsifying salt is used or when rework cheese is present at 0.0 or 5%, but no effect was noticed in meltability values if the 2.5% emulsifying salt was used with no rework cheese.