A new Na/K geothermometer equation has been developed. The temperature function is: [GRAPHICS] Concentrations are in mg/kg. The new improved geothermometer equation was developed by artificial neural networks. The normalized mean square error (NMSE) used in the new improved Na/K equation for temperatures ranging from 94 to 345 degreesC is 0.179, which is lower than the corresponding NMSE 0.226, 0.598, 0.656, 0.268, 0.328 and 0.225 for the equations of Arnorsson et al. (1983; Geochim. Cosmochim. Acta 47, 567-577), Truesdell (1975; Proc. 2nd UN Symposium), Tonani (1980; Proc. Adv. Eur. Geoth. Research, 2nd Symposium), Fournier (1979a; J. Volcanol. Geotherm. Res. 5, 1-16), Nieva and Nieva (1987; Heat Recovery Systems and CHP 7, 243-258) and Giggenbach (1988; Geochim. Cosmochim. Acta 52, 2749-2765), respectively. It is therefore strongly recommended to use this new improved Na/K geothermometer equation for geothermal exploration. Since the proposed Na/K geothermometer was developed on a purely empirical basis, it should not be applied outside the T-P-X range of calibration. (C) 2002 CNR. Published by Elsevier Science Ltd. All rights reserved.