Abstract Gaseous membrane permeation (MP) technologies have been combined with continuous‐flow isotope ratio mass spectrometry for on‐line δ 13 C measurements. The experimental setup of membrane permeation‐gas chromatography/combustion/isotope ratio mass spectrometry (MP‐GC/C/IRMS) quantitatively traps gas streams in membrane permeation experiments under steady‐state conditions and performs on‐line gas transfer into a GC/C/IRMS system. A commercial polydimethylsiloxane (PDMS) membrane sheet was used for the experiments. Laboratory tests using CO 2 demonstrate that the whole process does not fractionate the C isotopes of CO 2 . Moreover, the δ 13 C values of CO 2 permeated on‐line give the same isotopic results as off‐line static dual‐inlet IRMS δ 13 C measurements. Formaldehyde generated from aqueous formaldehyde solutions has also been used as the feed gas for permeation experiments and on‐line δ 13 C determination. The feed‐formaldehyde δ 13 C value was pre‐determined by sampling the headspace of the thermostated aqueous formaldehyde solution. Comparison of the results obtained by headspace with those from direct aqueous formaldehyde injection confirms that the headspace sampling does not generate isotopic fractionation, but the permeated formaldehyde analyzed on‐line yields a 13 C enrichment relative to the feed δ 13 C value, the isotopic fractionation being 1.0026 ± 0.0003. The δ 13 C values have been normalized using an adapted two‐point isotopic calibration for δ 13 C values ranging from −42 to −10‰. The MP‐GC/C/IRMS system allows the δ 13 C determination of formaldehyde without chemical derivatization or additional analytical imprecision. Copyright © 2009 John Wiley & Sons, Ltd.