The normalized intensity autocorrelation function g((2))(t) were obtained by dynamic light scattering for moderately concentrated entangled solutions of polyisobutylene in n-heptane at 25.0 degreesC and in isoamyl isovalerate (IAIV) at 25.0 degreesC (Theta). The obtained data have been successfully analyzed by the 'procedure X' familiar for determination of mechanical relaxation spectra on the basis of the recent theory for g((2))(t). The results have shown that while the mutual diffusion coefficient D increases in the n-heptane solutions and decreases in the IAIV solutions with increasing polymer mass concentration c, the friction coefficient for both solutions increases with c showing the same power-law behavior irrespective of the weight-average molecular weight M-w and solvent quality. It has been found that the instantaneous longitudinal modulus L-0 for n-heptane solutions increases in proportion to c(2), obeying the familiar relation for the plateau value (4/3)G(N) Of the longitudinal stress relaxation modulus, but L-0 for the IAIV solutions becomes progressively smaller than the values predicted from the relation with decreasing c. The terminal relaxation time tau(m) has been found to follow the power-law tau(m) infinity M-w(3.4) established by theological measurements.