During monthly synoptic surveys in a dry year, physicochemical and biological data were collected at 15 stations throughout the Tagus estuary. A Principal Component Analysis (PCA) was used to identify the major sources of variation in the data. The most important variations in the Tagus estuary were the spatial variations, mainly the longitudinal ones showing clearly the riverine and the sea water influenced zones of the estuary. The results also showed that a lateral variation was due to the local pollution as indicated by intense organic matter degradation and high rates of oxygen consumption. The seasonal variation related to temperature and the variation associated with the semidiurnal tidal cycle were also apparent. These, however, contributed less than the spatial variation to the total variability in the measured parameters of the Tagus estuary.