Photon correlation spectroscopy has been successfully employed to study the dynamics of the local segmental and sub-Rouse motion of a monodisperse polyisobutylene near and above the glass transition temperature. The separation between the maxima of the two peaks corresponding to these two viscoelastic mechanisms is obtained for several temperatures. The temperature dependence of the local segmental relaxation times as compared to the sub-Rouse modes relaxation times is noticeably stronger. The segmental correlation function can be well described by a Kohlrausch’s stretched exponential function with an exponent beta(alpha) = 0.56. At this time, photon correlation spectroscopy is the only experimental means that can determine the correlation of the local segmental motion of polyisobutylene directly. The small value of the coupling parameter of PIB, n(alpha) = 1 - beta(alpha) = 0.44, among amorphous polymers support the interpretation of its unusual viscoelastic properties. This value of n(alpha) directly determined from photon correlation spectroscopy, is consistent with the value deduced indirectly from other experiments.