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The rate of the transfer of deuterium from D 3 + to the three types (benzylic, aliphatic and aromatic) of bonds of 2-Ethylnaphthalene (and Methylnaphthalene) has been measured in the laboratory. The D/H ratio of each bond was calculated from the mass spectrum of the different isotopomers produced in the source of the GC mass spectrometer. The relations between the D/H ratio of the different isotopomers allow to calculate the internal isotopic fractionation factors (αi−j = (D/H)i/(D/H)j) between the 3 bonds: αBenzylic–Aromatic = 1.99 ± 0.38 and αAliphatic–Aromatic = 1.22 ± 0.39. Isotopic exchange rate constants are ⩾2.2 × 10−7±0.4 s−1, determined at room temperature for 10−3 g of 2-Ethylnaphthalene in contact with 5 × 1012 cm−3 of D 3 + . These results are compatible with those measured in the insoluble organic matter (IOM) isolated from the Orgueil meteorite ( Remusat et al., 2006 ), i.e., αBenzylic–Aromatic = 1.96 ± 0.05 and αAliphatic–Aromatic = 1.35 ± 0.05. Using the calculated dust and H2D+ densities from the models of deuterium chemistry in protostellar disks, the duration needed to enrich the IOM up to its measured D/H ratio is ⩽104±1 s. These laboratory results are in agreement with a late deuteration of the insoluble organic matter taking place in the solar T-Tauri disk by interaction of the organic dust with H2D+.