On the transects of the Amazon River made by the Alpha Helix in 1976 and 1977, an extensive suite of samples was collected for isotopic analyses. The water isotopes (18O/16O, D/H) were determined in atmospheric water vapour and in river, rain, and leaf waters. 13C/12C ratios were measured in the dissolved and atmospheric CO2. Determinations were made of 34S/32S and oxygen isotopes in dissolved sulphate. The effect of 'continentality' on the water isotopes is minor reflecting the large scale recycling by evapotranspiration from the huge area of forest within the basin. Variations in the isotopic abundances between 1976 (June--July, dry season) and 1977 (May--June, end of wet season) are consistent with the changes in meteorological conditions. The isotopic composition of the CO2, both atmospheric and dissolved, is dominated by biological effects. In 1976 the dissolved CO2 showed downstream variations from -14? at Iquitos in Peru to -22? in the lower reaches. In 1977, no systematic trend was apparent, the data ranging around -19?. The values for atmospheric CO2 decrease inland from marine values at the mouth to around -15? at Manaus. During the dry season (1976) the values in the interior, western basin were homogeneous at -20?. In the wet season there were considerable variations reflecting atmospheric instabilities with the average value being about -13?. The sulphur isotopic composition of the dissolved sulphate is remarkably uniform at around 7?. In 1977 the 18O values in the sulphate decreased systematically downstream from 8? in Peru to 3? at the mouth, consistent with a progressive redox-mediated exchange with water and dissolved oxygen. In 1977 the values increased to over 11?, apparently indicating exchange with a highly fractionated reservoir of dissolved oxygen perhaps in the semireducing environment of the flood plain lakes.