Herbicide mobility has been studied extensively in nonvolcanic soils but not in the volcanic soils that are important to the agricultural systems of many regions, including Pacific-Rim countries. Volcanic soils have unique properties that include high contents of organic carbon and soil water at 1500 kPa tension, low bulk density and saturation of bases, and the presence of amorphous clay. We hypothesized that the chemical and physical characteristics of these soils would affect the mobility of selected herbicides. Therefore, thin-layer chromatography was used to measure the relative mobility of atrazine, diethylamine salt of 2,4-D, and dicamba in volcanic soils of south-central Chile, including Arrayan silt loam (Typic Haploxerands), Santa Barbara silt loam (Typic Haploxerands), Arenales loamy sand (Dystric Xeropsamments), and weathered Collipulli clay loam (Xeric Palehumults) series. Dicamba was the most mobile herbicide in all of the soils; atrazine was the least mobile herbicide in the weathered Collipulli soil and in the surface horizons of the Arrayan and Arenales soils, but 2,4-D was the least mobile in the Santa Barbara soil. Herbicide mobility in the allophanic soils (Arrayan, Santa Barbara, and Arenales) could be estimated using the contents of soil organic matter and 1500 kPa water, and also soil pH. The relative mobility of atrazine was greater than expected in the Arrayan and Santa Barbara soils and was probably caused by the protonation and subsequent repulsion of atrazine by positively charged soil surfaces. The relative mobilities of 2,4-D and dicamba were smaller than expected in the Santa Barbara soil and were attributed to the retention of dissociated 2,4-D and dicamba by positively charged soil surfaces.