A sedimentological and geochemical study was performed on the travertines in the southern part of the Usak geothermal field, western Turkey, to assess the applicability of a fluvial tufa facies model in interpreting late Quaternary travertine deposits developed along the stream valleys that follow fault and fracture systems. Modern thermal (31 to 38 degrees C) springs are found on the floor of the valley between 480 m and 520 m above sea-level. The distribution and nature of travertine facies were determined from natural outcrops. Samples of the travertines and spring water were characterized using a range of geochemical and petrographic methods. Waterfall, slope and pool facies associations consist of various combinations of travertine facies and subordinate detrital facies. Waterfall and slope facies associations of the older deposits occur where the springs emerged onto a hillslope or topographic break. In contrast, the pool facies association developed in depressions or flat areas that were fed by thermal springs. The youngest generation (1.85 ka) precipitated at lower elevations than the older ones (147 to 153 ka). Stable carbon and oxygen isotope values of the Aksaz travertines range between +4.3 parts per thousand and +6.3 parts per thousand (Vienna Pee Dee Belemnite) and -12.6 parts per thousand and -7.2 parts per thousand (Vienna Pee Dee Belemnite), respectively. The high delta C-13 values suggest that the thermal waters were charged with isotopically heavy CO2 of deep origin. Based on palaeotemperature calculations, the temperatures of the palaeosprings are slightly higher (up to 44 degrees C) than the present equivalents, but sometimes the temperature is lower, probably due to mixing with the stream water. Although the thermal waters occasionally are impeded by fluvial activity, travertine precipitation occurs in the protected parts of the Aksaz Stream valley. This contribution highlights the applicability of the fluvial facies model for tufa for the interpretation of travertine deposits worldwide.