The theological behaviour of binary mixtures of polystyrene-block-polyisoprene (SI diblock) copolymers in the disordered state was investigated. For this study, two SI diblock copolymers having different molecular weights and block length ratios were synthesized via anionic polymerization, and then binary mixtures were prepared to measure their linear viscoelastic properties (namely, dynamic storage and loss moduli, G’ and G", as functions of angular frequency omega) at various temperatures. The disordered state of the block copolymers and their mixtures was determined by observing a critical temperature, often referred to as the order-disorder transition temperature (T-ODT), at and above which logarithmic plots of log G’ versus log G" become independent of temperature and have a slope of 2 in the terminal region. We found that log G’ versus log G" plots for the binary mixtures lie between those of the constituent components, and at a fixed value of G" the value of G’ increases monotonically with increasing blend composition. The composition dependence of the linear viscoelastic properties of the binary mixtures was investigated by preparing reduced plots using two different methods of determining reference temperature T-0 : (i) at an equal distance from the glass transition temperature of the polystyrene block (T-g,T-PS + 55 degrees C) in the respective block copolymer and also (ii) at an equal distance from the T-ODT of the respective block copolymer (T-0 = T-ODT + 10 degrees C). We found that the two different methods yielded very different composition dependence of the linear viscoelastic properties of the binary mixtures of SI diblock copolymers.