Methane (CH4), molecular hydrogen (H2), and water vapor (H2O) were measured concurrently on board the NASA ER-2 aircraft during the 1995--1996 Stratospheric Tracers of Atmospheric Transport (STRAT) and 1997 Photochemistry of Ozone Loss in the Arctic Region in Summer (POLARIS) campaigns. Correlations between these three main hydrogen reservoirs in the northern extratropical lower stratosphere are examined to evaluate H2O production from CH4 and H2 oxidation. The expected ratio of stratospheric H2O production (PH2O) to CH4 destruction (LCH4)=-1.973¿0.003 is calculated from an evaluation of CH4 and H2 oxidation reactions and the relationship between H2 and CH4 mixing ratios measured during STRAT. Correlations between H2O and CH4 were tight and linear only for air masses with mean ages ≥3.8 years, restricting this analysis predominantly to latitudes between 40¿ and 90 ¿N and potential temperatures between 470 and 540 K. The mean observed ΔH2O/ΔCH4 (-2.15¿0.18) is in statistical agreement with the expected PH2O/LCH4. The annual mean stratospheric entry mixing ratio for H2O calculated from this slope is 4.0¿0.3 ppm. The quantity H2O+2⋅CH4 is quasi-conserved at 7.4¿0.5 ppm in older air masses in the northern extratropical lower stratosphere. Significant departure of H2O+2⋅CH4 from the mean value is a sensitive indicator of processes which influence H2O without affecting CH4, such as dehydration in a polar vortex or near the tropical tropopause. No significant trend is observed in ER-2 aircraft data for H2O+2⋅CH4 in the lower stratosphere from 1993 through 1997. ¿ 1999 American Geophysical Union