New He and C relative abundance, isotope and concentration results from nine geothermal locations situated along an 800-km transect of the North Anatolian Fault Zone (NAFZ), Turkey, that were monitored during the period November 2001-November 2004, are reported. The geothermal waters were collected every 3-6 months to study possible links between temporal geochemical variations and seismic activity along the NAFZ. At the nine sample locations, the He isotope ratios range from 0.24 to 2.3R(A), delta C-13 values range from 4.5 to +5.8 parts per thousand, and CO2/He-3 ratios range from 5 x 10(9) to 5 x 10(14). The following geochemical observations are noted: (1) the highest He-3/He-4 ratios are found near the Galatean volcanic region, in the central section of the NAFZ, (2) at each of the nine sample locations, the He-3/He-4 ratios are generally constant; however, CO2/He-3 ratios and He contents both show one order of magnitude variability, and delta C-13 values show up to similar to 4 parts per thousand variability, and (3) at all locations (except Resadiye), delta C-13 values show positive correlations with CO2 contents. The results indicate that at least three processes are necessary to account for the geochemical variations: (1) binary mixing between crustal and mantle-derived volatiles can explain the general characteristics of He-3/He-4 ratios, delta C-13 values, and CO2/He-3 ratios at the nine sample locations; (2) preferential degassing of He from the geothermal waters is responsible for variations in CO2/He-3 values and He contents at each sample location; and (3) CO2 dissolution followed by calcite precipitation is responsible for variations in CO2 contents and delta C-13 values at most locations. For each of the geochemical parameters, anomalies are defined in the temporal record by values that fall outside two standard deviations of average values at each specific location. Geochemical anomalies that may be related to seismic activity are recorded on June 28, 2004 at Yalova, where a M = 4.2 earthquake occurred 43 days earlier at 15 km distance from the sample location, and on April 7, 2003 at Efteni, where a M = 4.0 earthquake occurred 44 days later at a distance of 12 km. At both locations, the sampling periods containing geochemical anomalies were preceded by an increase in M >= 3 earthquakes occurring within 60 days and less than 40 km distance. (C) 2010 Elsevier Ltd. All rights reserved.