The linear viscoelastic properties of 1,4-polyisoprene (PI; M-W = 504 000) were measured by combined mechanical and dielectric spectroscopies. For local segmental motion, the respective mechanical and dielectric relaxation times, although differing substantially in magnitude, have identical temperature dependencies. The shape of the segmental relaxation function itself was sensitive to temperature. The data, obtained over a 160 degree range of temperatures, could be reduced to yield ostensibly satisfactory master curves; however, detailed analysis reveals the usual breakdown of time-temperature superpositioning in the glass transition zone. Given the near equivalence of the respective breadths of its terminal and segmental dispersions, thermorheological complexity in PI is unanticipated by theory. In the plateau zone, the mechanical spectrum of PI exhibits power law behavior, with an exponent (0.21) close to a proposed universal value.