Natural rubber (NR) was blended in various ratios with 29 kinds of tackifier resins. Miscibilities of all the blend systems were illustrated as phase diagrams. From these blend systems, we selected 8 systems having typical phase diagrams (completely miscible, immiscible, lower critical solution temperature [LCST] types) and carried out measurements of shear creep resistance (holding power). Holding time was recorded as required time for the pressure-sensitive adhesive (PSA) tape under shear load to completely slip away from the adherend. Holding time of miscible PSA systems tended to decrease as the tackifier content increased. This is attributable to a decrease in plateau modulus of the PSA with increasing tackifier content. There was rather large difference in holding time by tackifier among the miscible PSA systems; the reason for this is also considered to be a difference in plateau modulus. Holding time of an immiscible PSA system scarcely changed by tackifier content. But in another immiscible system, holding time tended to increase with increasing tackifier content. In fact, in the case of immiscible PSAs, the effect of tackifier content on holding time was different from tackifier to tackifier. This may be caused by difference in extent of phase separation. (C) 2000 John Wiley & Sons, Inc.