The composition of commercially available ochres and siennas were studied by thermal analysis (TG and DTG) and XRD analysis. The significant parameters found on the thermoanalytical curves enable the classification of the ochres tested. Depending on the matrix of the sample, ochres can be classified into: ochres containing kaolinite and ochres containing sulphate. A relationship between the matrix and the source of the sample may be established. (C) 2003 Elsevier B.V. All rights reserved.