Vapour/liquid fractionation of rare earth elements and yttrium (REY), Na+, K+, NH4+, HCO3-, SO42-, Cl- and borate in geothermal fluids from the Piancastagnaio geothermal field (Mt. Amiata geothermal area, Tuscany, central Italy) were evaluated based on the chemistry of collected liquids and condensed vapours. Apparent vapour-liquid partitioning factors (D-app(V/L)) of REY vary from 0.3 to 0.01. These factors are much higher than those of Na (<0.001) and K (similar to 0.001). Volatile components are ion pairings such as NH4HCO3 degrees, NH4Cl degrees, NaHCO3 degrees, CaSO4 degrees and REYO(HCO)(3)degrees double left right arrow REY(OH)CO3 degrees. In vapour, REY and NH4+ are negatively correlated. High and low D-app(V/L)(REY) indicate variations of NH4+ concentrations in liquids. The results of this study are relevant to the understanding of element migration and deposition under hydrothermal boiling conditions. (C) 2009 Published by Elsevier Ltd.