[en] Full text: We describe a portable Fourier Transform InfraRed (FTIR) spectrometer for laboratory and field measurements of δ¹³C in CO₂ and δD in water vapour at ambient atmospheric levels. The instrument is based on a commercial 1 cm^-1 resolution FTIR spectrometer fitted with a mid-IR globar source, 26 m multipass white cell and thermoelectrically-cooled MCT detector operating between 2000 and 7500 cm^--1. δD in water vapour is measured in whole air passed at 1-2 L min^-1 through the cell in real time without any pre-treatment. For δ¹³C measurements the sample airstream is dried to < 20 μmol mol^-1 to avoid interference from water vapour. An inlet selection manifold allows automated sequential analysis of samples from up to 12 inlet lines, with typical measurement times of 4-5 minutes per line. The spectrometer, inlet sampling sequence, real-time spectrum analysis, data logging and real-time display are all under the control of a single program running on a laptop PC, and can be left unattended for continuous measurements over periods of days to weeks. Selected spectral regions of typically 100-200 cm^-1 width are analyzed by a least squares fitting technique to retrieve concentrations of trace gases and isotopologues. Typical precision is 1-2 %_o for δD and 0.1 - 0.2 %_o for δ¹³C. Calibration and performance are described in more detail in an associated poster. The collected spectra also provide simultaneous analysis of concentrations of CO₂, CH₄, CO and N₂O in the analyzed air samples with high precision, typically 0.1%. Performance of the FTIR analyzer will be illustrated with results from a recent field campaign in which we measured vertical profiles of δD in water vapour and δ¹³C in CO₂ on a 70 m tower in a eucalypt forest in SE Australia. Hourly 7-point profiles were obtained continuously for 3 weeks interspersed with measurements from soil and leaf chambers. The results, combined with a multilayer ecosystem model of water and carbon exchange, are described in detail in the paper by Haverd et al. (this conference). (author)