Chemical and isotopic (He, Ar and C) compositions have been measured for bubbling gases discharging from two closely paced earthquake prediction wells in Guangdong, SE China. Observed He-3/He-4 ratios (R/R(A) ranging from 1.1 to 4.9) indicate the occurrence of a mantle component that has been diluted by a crustal radiogenic He-4 component. delta(13)C values of CO2 suggest a different origin in the two wells: magmatic for ZK#1 (-6.8 to -8.4 parts per thousand) and biogenic for ZK#7 (-14.5 to -18.2 parts per thousand). Observed Ar-40/Ar-36 and air-corrected N-2/Ar ratios suggest that nitrogen and argon of both wells are mostly meteoric. While the CO2/He-3 ratios (similar to-1x10(9)) in ZK#1 are close to that of MORE (Middle Oceanic Ridge Basalts), the ZK#7 is characterized by much lower CO2/He-3 ratios (similar to 1x10(7)) with lower CO2 concentration, The latter cannot be ascribed in terms of a simple mixing of the two components but has to be explained by removal of CO2, probably caused by calcite precipitation. Alternatively, CO2 removal from and He-4 addition to the magmatic component may be responsible for the coupled variations of He-3/He-4 and CO2/He ratios in the two wells, which is most likely controlled by the degree of gas-water-rock interaction.