More than 450 air samples that were collected in the upper troposphere – lower stratosphere (UTLS) region by the CARIBIC aircraft (Civil Aircraft for the Regular Investi-gation of the atmosphere Based on an Instrument Container) have been analyzed for molecular hydrogen (H 2) mixing ra-tios (χ (H 2)) and H 2 isotopic composition (deuterium con-tent, δD). More than 120 of the analyzed samples contained air from the lowermost stratosphere (LMS). These show that χ(H 2) does not vary appreciably with O 3 -derived height above the thermal tropopause (TP), whereas δD does increase with height. The isotope enrichment is caused by H 2 production and destruction processes that enrich the stratospheric H 2 reservoir in deuterium (D); the exact shapes of the profiles are mainly determined by mixing of stratospheric with tro-pospheric air. Tight negative correlations are found between δD and the mixing ratios of methane (χ(CH 4)) and nitrous oxide (χ (N 2 O)), as a result of the relatively long lifetimes of these three species. The correlations are described by δD[‰] = −0.35 · χ(CH 4)[ppb]+768 and δD[‰] = −1.90 · χ(N 2 O)[ppb]+745. These correlations are similar to previ-ously published results and likely hold globally for the LMS. Samples that were collected from the Indian subcontinent up to 40 • N before, during and after the summer monsoon season show no significant seasonal change in χ(H 2), but δD is up to 12.3 ‰ lower in the July, August and Septem-ber monsoon samples. This δD decrease is correlated with the χ(CH 4) increase in these samples. The significant corre-lation with χ (CH 4) and the absence of a perceptible χ(H 2) increase that accompanies the δD decrease indicates that mi-crobial production of very D-depleted H 2 in the wet season may contribute to this phenomenon. Some of the samples have very high χ (H 2) and very low δD values, which indicates a pollution effect. Aircraft engine exhaust plumes are a suspected cause, since the effect mostly occurs in samples collected close to airports, but no similar signals are found in other chemical tracers to support this. The isotopic source signature of the H 2 pollution seems to be on the low end of the signature for fossil fuel burning.