Phase diagrams of blends consisting of poly(3-n-alkylthiopene) having four different n-alkyl side chain lengths (n = 4, 6, 8,12) and polystyrene (PS) were obtained by turbidity measurement. All of the P3AT/PS blends employed in this study exhibited the upper critical solution temperature (UCST) type phase behavior. From n=4 [poly(3-butylthipene)] to n=6 [poly(3-hexylthipene)], the miscibility between P3AT/PS blends decreased, but with further increase in the value of n, the miscibility increased. Thus, the miscibility of the P3HT/PS blend becomes the least among four blend pairs. This interesting phase behavior depending on n was successfully illustrated via the combination of Monte Carlo simulation and molecular dynamics. (C) 2009 Elsevier Ltd. All rights reserved.