Ten samples of olive mill wastewater (OMW) taken from different mills In southern Spain and other ten of OMW sludges from evaporation ponds were analysed. The aim was to study the composition of these wastes and to find relationships which would make it possible to use easily determinable parameters to ascertain their composition. Compared with other organic wastes, these materials had a high potassium concentration, a similar organic matter content and notable levels of nitrogen; phosphorus, calcium, magnesium and iron. The highest potassium concentrations were observed in the OMWs, while the sludges showed higher levels of the other nutrients, especially iron. The dry matter of the OMWs was significantly correlated with most of:the parameters studied but, in the sludges, the only correlation was between the ash content and the total organic carbon and total nitrogen concentrations. The regression equations obtained permitted a rapid. characterization of the OMWs from their dry matter content.