We investigated the compatibility of blends of 1,4-rich polyisoprene (1,4-PI) and poly(4-n-alkylstyrene)s with six kinds of n-alkyl side groups, that is, methyl, ethyl, propyl, butyl, hexyl, and octyl focusing on carbon number of alkyl groups. Poly(4-methylstyrene)/1,4-PI blend was turned out to be immiscible at all temperature range adopted in this work and poly(4-ethylstyrene)/1,4-PI blend revealed UCST type phase behavior, while the others were found to be compatible. The phase diagrams of poly(4-ethylstyrene)/1,4-PI blends were obtained by optical microscopy, and the temperature dependence of the Flory-Huggins interaction parameter chi has been estimated to be chi = -0.036 + 24/T by applying lattice theory, where T is the absolute temperature. From this relationship chi value at room temperature (298 K) was calculated to be 0.045, the value is reasonably low for miscible polymers system. (C) 2017 Wiley Periodicals, Inc.