Bromide (Br^-) was used to successfully trace the infiltration of rainfall through a sandy soil in northeast Thailand. A method was developed to measure the Br- concentration in extracted water from soil samples in order to determine the vertical distribution of the Br^- concentration in soil water. Our experimental results revealed that Br^- moved downward with a piston flow and that the increase in the amount of water in the soil above the depth of the peak Br^- concentration equaled the increase in soil moisture caused by the infiltration. Using our method, we found that the largest amount of rainfall infiltration occurred during September. The amount of soil water above the peak Br^- concentration was calculated to be about 0 mm during the period between June 27, 2003 and August 7, 2003; about 100 mm during the period between June 27, 2003 and September 10, 2003; and about 80 mm during the period between June 27, 2003 and February 12, 2004.