An investigation and assessment was carried out of thallium pollution of the soils around a slag dump of a Sulphuric Acid Plant in Guangdong Province with the enrichment index method. The slag contains thallium pyrite. Results of the investigation showed that the soils around the slag dump were obviously polluted with Tl. The pollutant of thallium was mainly concentrated in the surface layer (0 ~ 16.5 cm) of the soils and its concentration drops rapidly and vertically along the profile. However, the Tl adsorption in the surface layer has not yet reached its saturation state, so it keeps on accumulating in the layer. Results of the enrichment index method also showed that the soil (0 ~ 6 cm) underneath the slag pile was polluted with Tl to a moderate level, while the soil layer below 15 cm in the profile was not contaminated. In comparison with the soil underneath the slag pile, the soil in the periphery of the pile had its surface layer (0 ~ 1 cm) already severely polluted with Tl, and its 1 ~ 5 cm layer, moderately polluted, but the soil layer below 15 cm remains clear of Tl pollution. It is quite obvious that the enrichment index method is applicable to analysis of soil Tl pollution level, but it is not good at exploring status of its existence, its ability of translocation, and its bioavailability in the soil.