Abstract A series of chemical analyses and dry density tests of rhyolitic tuff and granitic samples have been carried out. The test results have been interpreted using a newly developed theoretical weathering model for calculating a mobility index of major elements in rock and soil. It is found that this index is useful for describing changes of rock and soil during weathering and classifying the degrees of weathering. Based on the theoretical model, a new parameter called the volume index, which combines chemical and physical data, is suggested. This index appears to be a good indicator for classifying the degree of weathering in Hong Kong. A quantitative classification scheme is suggested.