Volume flow of polyisobutylene (PIB) (Mn= 1,283,000 and Tg= 202 K) has been measured in an Instron Capillary Rheo-meter. Elastic modulus in longitudinal compression, longitudinal volume viscosity, and retardation times are described at temperatures from about 295 K to 333 K, and at compression rates of ca. 1.00 to 200.00 × 10−5s−1The longitudinal volume viscosity decreases with increasing compression rate, decreasing volume deformation (this behavior being a typical nonequilibrium one), and decreasing temperature, the volume flow activation energy amounting to some 19.19 kJ/mol.