Ceria-supported ruthenium (Ru/CeO2) was found to be an effective catalyst for linear-selective dimerization of styrenes. The dimerization of styrene in the presence of Ru/CeO2 together with ethanol and formalin in mesitylene at 150 °C gave (E)-1,4-diaryl-1-butene in 63 % yield with high linear selectivity (95 %). The presence of ethanol and/or formaldehyde is indispensable. Whereas a zirconia-supported ruthenium showed moderate activity, catalysts supported on other metal oxides were not effective at all. The hot filtration tests indicated that the Ru/CeO2 catalyst acts heterogeneously. The leaching of ruthenium species into the solution was negligible according to analysis by using inductively coupled plasma atomic emission spectroscopy. The regioselectivity can be switched through the choice of a suitable solvent: the reactions in N,N-dimethylacetamide or N-methyl-2-pyrrolidone gave the corresponding branched dimers in good yields with complete regioselectivity.