Elemental composition characterization of 19 clay samples collected from 12 localities around Conimbriga and Santa Olaia archaeological sites were carried out by neutron activation analysis, in order to identify the sources of clay used to manufacture the fine grey pottery recovered at those sites. Some samples have shown a correlation with the Santa Olaia ceramics, and only one was found to be correlated with the Conimbriga ceramics. To determine the relationships between ceramics and clays, petrographic analysis of selected sherds of Conimbriga and Santa Olaia pottery were made by thin section and also by using a scanning elecron microscope. The results of the petrographic analysis agree quite well with those of chemical analysis. The influence of the separation of fractions of different sizes from the clay samples showed that the resulting variation of composition for the fractions less than38mum did not improve the above-mentioned correlation.