The phase behavior of statistical copolymers composed of (4-tert-butylstyrene) (B) and (4-tert-butoxystyrene) (O), abbreviated as s-BO, with polyisoprene (I) was investigated by optical microscopic (OM) observation and small-angle neutron scattering (SANS) measurements. It has been known that B/I blend shows lower critical solution temperature (LCST) type phase diagram, while O/I blend has upper critical solution temperature (UCST) type one. Several blends of s-BOs having mol fraction of B, m(B), comparable to 0.50, with I showed both UCST and LCST type phase diagram. Furthermore, UCST type phase behavior was observed for blends having small m(B), while LCST type one was for that of large m(B) at all used temperatures. Hence, the phase behavior of s-BO/I blend can be understood as a result of the competition of two interactions having opposite temperature dependence. (C) 2009 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys 47: 2272-2280, 2009