Measurements of δ13C and δD of atmospheric CH4 from whole air samples collected in the upper troposphere and lower stratosphere aboard the NASA ER-2 aircraft during the SOLVE (2000), POLARIS (1997), and STRAT (1996) campaigns are reported. Samples cover latitudes from 1¿S to 89¿N and altitudes from 11 to 21 km, providing CH4 mixing ratios that range from 1744 to 716 ppbv. Measurements of isotope ratios were made by continuous-flow gas chromatography isotope ratio mass spectrometry which provides high-precision analyses on 60 ml aliquots of air. These measurements comprise the first upper atmosphere isotopic CH4 data set to date using this technique and the most extensive with respect to latitude and season in any case. Values of δ13C-CH4 on the V-PDB scale range from -47.28? near the tropical tropopause to -34.05? in the high northern latitude stratosphere. Values of δD on the V-SMOW scale range from -90.9? to +26.4?. Correlations of isotope ratios with CH4 mixing ratios show enrichment in the heavy isotopes as CH4 mixing ratios decrease due to kinetic isotope effects associated with oxidation by reaction with OH, Cl, and O(1D). Empirical fractionation factors are found to be highly dependent on the range of CH4 mixing ratio considered, increasing with decreasing mixing ratio. Systematic nonlinearity in a Rayleigh fractionation model suggests a range of stratospheric fractionation factors, αCstrat = 1.0108 ¿ 0.0004 to 1.0204 ¿ 0.0004 (2σ) and αHstrat = 1.115 ¿ 0.008 to 1.198 ¿ 0.008 (2σ), from high to low CH4 mixing ratio, respectively. The variation in α over the range in mixing ratios reflect changes in partitioning between CH4 sink reactions in different regions of the stratosphere. In Part 1, these new high-precision observations are discussed and compared with other stratospheric and tropospheric isotope measurements. In Part 2 <McCarthy et al., 2003> the observations are compared with 2-D model results, and implications for the kinetic isotope effects for reactions with OH, Cl, and O(1D) are discussed.