Solvolysis of cis- and trans-α-bromo-4,4′-dimethoxystilbenes (3) and (4) in 80% and 90% EtOH and in AcOH gives mainly bis-4-methoxyphenylacetylene (8). The first order kcis: ktrans reactivity ratios in aqueous ethanol are 43·5–49 in the presence of sodium acetate and 19·7 in the acetolysis. With 0·5M-NaOH in 80% EtOH kcis:ktrans= 0·06 where kcis is a first-order coefficient and ktrans is the second-order coefficient for the E2 reaction. Acetolysis of cis- and trans-αβ-dibromo-4,4′-dimethoxystilbenes (5) and (6) is slower than those of compounds (3) and (4). Values of k(3)/k(5)= 187, k(4)/k(6)= 3·3 and k(5)/k(6)= 0·37–0·41 were calculated. The products with sodium acetate are 4,4′-dimethoxybenzil, 4,4′-dimethoxybenzoin acetate, deoxy-4,4′-dimethoxybenzoin, and the acetylene (8), but in the presence of AgOAc, trans-α-acetoxy-β-bromo-4,4′-dimethoxystilbene (7) was formed. The acetolysis of all the compounds showed common ion rate depression and that of compound (7) gives similar products to those of (5) and (6). The mechanism of the reaction of compound (4) in the presence of sodium hydroxide is suggested to be E2 while all the other reactions are mechanistically SN1–E1. The rate difference between compounds (3) and (4) in the E1 process is ascribed mainly to a rate enhancement of compound (3) due to ground-state steric interactions. No evidence for β-bromine participation was observed. The structure and the selectivity of the intermediate vinyl cations are discussed.