Henry's law constants of 1-butene, 2-methylpropene, trans-2-butene, and 1,3-butadiene in methanol in the temperature range of 374-490 K are experimentally obtained. A similar method to a gas stripping method is applied to measure the Henry's law constants at high temperatures up to the critical point of methanol. The rigorous formula for evaluating the Henry's law constants from these measurements is applied to the data reduction for these highly volatile mixtures. By using this formula, a volume effect of vapor phase is discussed. The plot of Henry's law constants versus temperature goes through a maximum and approaches an unique point at the critical temperature of methanol. The fugacity coefficient of the solute in the vapor phase at infinite dilution and the infinite dilution activity coefficient of the solute in liquid phase are evaluated from these experimental data. (c) 2006 Elsevier B.V. All rights reserved.