A system capable of measuring scalar fluxes with a relaxed eddy accumulation (R.E.A.) method aboard research vessels was developed. It accumulates air samples at a constant flow rate in two containers according to the sign of the instantaneous, true vertical wind velocity and then, at the end of the conditional sampling period, it measures the mean concentration of the gas of interest in each reservoir. The flux is proportional to the standard deviation of the vertical wind speed as well as to the difference in the mean concentration of the scalar associated with updrafts and downdrafts. This is an usual method to derive non reactive scalar fluxes over the continent but it remains difficult to implement aboard mobile platforms due to the necessity to compute in real time the vertical wind velocity, corrected from the platform motions and filtered to remove any bias on it. A simplified version of the system was tested over a maize field so as to estimate the CO2 and nitrogen fluxes during the ESCOMPTE experiment (June 2001). In this campaign devoted to pollution and transport of chemical species, the computation of the true vertical wind velocity was switched off in the real time algorithm of air selection. The complete system was implemented aboard a research vessel during the POMME experiment. This campaign occurred in the Northern Atlantic Ocean, off the Azores coasts between January and May 2001. The validation of this prototype is conducted through comparisons with fluxes simultaneously computed by another technique (bulk latent heat flux onboard the ship during POMME; latent heat flux measured eddy correlation technique and nitrogen flux by profile method above the maize parcel during ESCOMPTE).