The relation between cosolvent and dissolution of n-alkanes from TPHs (Total Petroleum Hydrocarbons) contaminated soil has not been well studied.  The objective of this study was to investigate the effect of cosolvent on the dissolution of n-alkanes from the soil particles to the aqueous phase.  TPHs contaminated soils were sampled from the site for former oil storage tank for over 50 years, in Gyeonggi, Korea.  The dissolution concentration of each n-alkane from the contaminated soil was obtained as a function of methanol volume fraction (f_c = 0.0, 0.2, 0.4) with 5 mM CaCl₂ solution.  In the batch experiment, 25 g contaminated soil samples were mixed with 25 mL methanol+CaCl₂ solution.  The mixtures were shaken for 3 hr ~ 72 hr at room temperature (25±2 °C).  The quantification of dissolved n-alkanes was carried out with GC/FID, using TRPH Standard as the external standard.  The concentration of n-alkanes with even carbon number between C12 ~ C32 was measured.  The dissolution data collected as a function of time were found to follow the first order kinetics.  In the first 24 hr, almost samples showed fast dissolution of n-alkanes.  Subsequently, the dissolution proceeds at a much slower rate before reaching a stable dissolution plateau.  When f_c is higher (f_c = 0.4), the total dissolved concentrations of n-alkanes were higher than others (f_c = 0.0, 0.2).  Also, the time to reach equilibrium was the shortest in the highest f_c solution (f_c = 0.4).  In the soil matrix, soil solution which has cosolvent can augment the mobility of n-alkanes by increasing dissolution n-alkanes from the soil particles.