This study focuses on the basic test and evaluation requirements necessary for a successful competency assessment of the contaminant attenuation capability of clay soils. Two clay soils from South Wales were chosen for study. Of particular interest is the problem of specification of ‘necessary and sufficient tests’ for proper assessments of competency. The tests and evaluation protocols leading towards confident competency assessment of the candidate clay soils are developed. Soil column leaching studies, together with results from analyses of the physicochemical and mineralogical characteristics of the soils, are used to determine clay soil contaminant attenuation capability. The results demonstrate that reliance on a limited amount of information can lead to some inappropriate premature interpretations that can prove to be harmful. In a situation where attenuation of dissolved metals is important, only one of the soils tested is considered to be an adequate clay soil for a clay engineered barrier.