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We present gas mass fractions of 38 massive galaxy clusters spanningredshifts from 0.14 to 0.89, derived from Chandra X-ray data and OVRO/BIMAinterferometric Sunyaev-Zel'dovich Effect measurements. We use three models forthe gas distribution: (1) an isothermal beta-model fit jointly to the X-raydata at radii beyond 100 kpc and to all of the SZE data,(2) a non-isothermaldouble beta-model fit jointly to all of the X-ray and SZE data, and (3) anisothermal beta-model fit only to the SZE spatial data. We show that the simpleisothermal model well characterizes the intracluster medium (ICM) outside ofthe cluster core in clusters with a wide range of morphological properties. TheX-ray and SZE determinations of mean gas mass fractions for the 100 kpc-cutisothermal beta-model are fgas(X-ray)=0.110 +0.003-0.003 +0.006-0.018 andfgas(SZE)=0.116 +0.005-0.005 +0.009-0.026, where uncertainties are statisticalfollowed by systematic at 68% confidence. For the non-isothermal doublebeta-model, fgas(X-ray)=0.119 +0.003-0.003 +0.007-0.014 and fgas(SZE)=0.121+0.005-0.005 +0.009-0.016. For the SZE-only model, fgas(SZE)=0.120 +0.009-0.009+0.009-0.027. Our results indicate that the ratio of the gas mass fractionwithin r2500 to the cosmic baryon fraction is 0.68 +0.10-0.16 where the rangeincludes statistical and systematic uncertainties. By assuming that cluster gasmass fractions are independent of redshift, we find that the results are inagreement with standard LambdaCDM cosmology and are inconsistent with a flatmatter dominated universe.Comment: ApJ, submitted. 47 pages, 5 figures, 8 table

X‐Ray and Sunyaev‐Zel’dovich Effect Measurements of the Gas Mass Fraction in Galaxy Clusters