A “real-time” eddy covariance (EC) system (EdiSol) was developed to measure fluxes of CO₂, momentum, sensible heat and latent heat of a boreal black spruce stand as part of the BOReal Ecosystem-Atmosphere Study (BOREAS). BOREAS is a multi-scale study using satellites, aircraft, tower and leaf-scale measurements over the major vegetation types of the Canadian boreal forest. The measurements reported in this thesis are an essential component of BOREAS being the stand scale (tower) measurements over one of the major vegetation types. EdiSol uses a commercially available three-axis ultrasonic anemometer and a closed path infrared gas analyser (air is ducted from close to the anemometer to the optical bench). Software was written to acquire the output data from the anemometer and gas analyser and concurrently calculate CO₂, water vapour, momentum, sensible heat and latent heat fluxes. The high frequency, primary, data is stored for further analysis. The calculations made by the software and the instruments and their connection are described. The corrections for frequency response of the system and tests of the software output are presented. The observations were made from 25 March to 28 November 1996 over the black spruce stand. There was no CO₂ assimilation when soil temperature at 5 cm and 10 cm depth was less than ca 0°C (corresponding to an air temperature of -3°C on average). Under these conditions, carbon exchange averaged 0.8 g C m^-2day^-1 (0.77 mmol CO₂ m^-2 s^-1). Mean half-hourly net ecosystem flux reached -14 mmol m^-2 s^-1 in June during the day and 10 mmol m^-2 s^-1 at night when the atmosphere was well mixed (u_* > 0.4 m s^-1). The optimum temperature for CO₂ assimilation was 30°C, close to the maximum air temperature observed at 26 m. When the soil temperature above 10 cm depth was less than 0°C assimilation was zero but quantum efficiency of assimilation (a) reached 0.66 in July.