A survey of the toxicity of soils from the several industrial zones of the Spanish Basque Country was undertaken in order to identify the relationship between chemical contamination and toxicity. The measured effect in the solid and liquid‐phase Microtox toxicity test was correlated with the chemical parameters to determine the origin of the toxicity effect. Results indicate the higher sensitivity of solid‐phase bioassay. In a comparative study with the liquid‐phase assay it was found that due to different solubility of each contaminant in water the test on the extracts represents only a part of multiple contamination. Moreover water elutriation could underestimate the types and concentrations of organic contaminants present.