The adsorption coefficient, Kd (ml/g), is important as a retaining force in slowing down MeBr transport through the soil. There are a few published measured or estimated Kd and Koc values for MeBr. The reported Koc ranges from 9 to 22 (Briggs, 1981; Karickhoff, 1981; Rao et al., 1985), which corresponds to a Kd of 0.09 to 0.22 in a soil with 1% organic carbon. Arvieu (1983) measured MeBr adsorption and desorption, and found different characteristics for soil with different organic matter contents. In organic matter-poor soils, the adsorption of MeBr is very weak unless the soil is very dry. In organic matter-rich soils, the adsorption is considerably greater. The same author also noted that the adsorbed MeBr became resistant to desorption. Gan and Yates (1996) observed that degradation of MeBr during the equilibration in adsorption studies might have contributed to the observed increased adsorption in soils with high organic matter content. A noticeable fraction of the spiked MeBr was degraded to Br– during a 16-h shaking in organic matter-rich soils. This phenomenon may be also responsible for the irreversibility found in MeBr desorption isotherms (Arvieu, 1983). After correcting for the degraded fraction, MeBr adsorption became negligible in all the tested soils (Gan and Yates, 1996). MeBr can be considered to be a nonadsorbing chemical in soil with normal water content.