This paper presents the results of a parametric study done on a single stage finger-type packed-bed membrane reactor (PBMR) used for heavy water vapor dedeuteration. Thanks to mass spectrometer analysis of streams leaving the PBMR, speciation of deuterated species was achieved. Measurement of the amounts of each molecular component allowed the calculation of reaction quotient at the packed-bed outlet (i.e. retentate). They highlighted that isotopic exchange reactions occurring on the catalyst particles surface are not thermodynamically equilibrated. Moreover, variation of the heavy water content in the feed exhibits competition between permeation and conversion kinetics.