Polyisobutylene (PIB) was the first polymer whose viscoelastic behavior was thoroughly studied. The measurements made worldwide on the PIB sample distributed from the National Bureau of Standards were compiled and analyzed by Bob Marvin. The superposition of the data substantiated the time–temperature reduction process, which was suggested by Herbert Leaderman, first put into practice by Arthur Tobolsky, and given a theoretical foundation by John Ferry. Polymers that behaved in a like manner were called thermorheologically simple by A. J. Staverman and F. Schwarzl. Subsequently the behavior of PIB has been found to be the exception rather than the rule. The various deviations for this simplicity are briefly reviewed and PIB is reexamined.