The behavior of poly(ethylene terephthalate) (PET) oligomers dissolved in refrigerant- oil mixtures has been elucidated, following the rates of the quantities of dissolved oligomers in reciprocating compressors. The quantities of oligomers extracted from PET films increase with increasing operating temperatures. Oligomers dissolved in refrigerant-oil mixtures yield deposits at discharge valves when compressors are operated at .high enough temperatures. The extraction rates of oligomers from PET films with refrigerant-oil mixtures have been also studied under various conditions. It is shown that the quantities of extracted oligomers increase in proportion to the square root of time. Other factors which affect the extraction rates of oligomers, such as temperature, the kind of refrigerant, the kind of PET, etc., are discussed.