The San Matías Gulf, located between 40 47' S and 42 13' S on the Atlantic coast of South America, with an area of approximately 18 000 km², is the second largest in Argentina. This gulf has been studied by means of historical data obtained in oceanographic campaigns carried out before 1990 and with satellite images of low-resolution (1000 m) NOAA-AVHRR obtained from 1985. The analysis of this information permitted to infer some environmental characteristics related to temperature, salinity, productivity and circulation. The objective of the present work is to analyze LANDSAT-TM and ETM+ data from 1997-2003. Approximately 30 free of clouds images, obtained in different times of the year, were processed. The thermal channel allowed to obtain a detailed distribution of surface temperature, to identify thermal fronts and to analyze its variability along the year. Also, this channel and channel 1 permit to observe the influence, in the southern part of the gulf, of the water coming from the continental shelf and San José Gulf. The results obtained did not only contributed to corroborate the environmental characteristics already inferred previously from cruise data, but also, to identify others, so far unknown