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The relative abundances of atomic and molecular species in planet-formingdisks around young stars provide important constraints on photochemical diskmodels and provide a baseline for calculating disk masses from measurements oftrace species. A knowledge of absolute abundances, those relative to molecularhydrogen (H2), are challenging because of the weak rovibrational transitionladder of H$_{2}$ and the inability to spatially resolve different emissioncomponents within the circumstellar environment. To address both of theseissues, we present new contemporaneous measurements of CO and H2 absorptionthrough the "warm molecular layer" of the protoplanetary disk around theClassical T Tauri Star RW Aurigae A. We use a newly commissioned observing modeof the Hubble Space Telescope-Cosmic Origins Spectrograph to detect warm H2absorption in this region for the first time. An analysis of the emission andabsorption spectrum of RW Aur shows components from the accretion region nearthe stellar photosphere, the molecular disk, and several outflow components.The warm H2 and CO absorption lines are consistent with a disk origin. We modelthe 1092-1117A spectrum of RW Aur to derive log10N(H2)~=~19.90$^{+0.33}_{-0.22}$ at T$_{rot}$(H2) ~=~440~+/-~39 K. The CO$A$~--~$X$ bands observed from 1410-1520A are best fit by log10N(CO)~=~16.1~$^{+0.3}_{-0.5}$ at T$_{rot}$(CO) ~=~200$^{+650}_{-125}$ K.Combining direct measurements of the HI, H2, and CO column densities, we find amolecular fraction in the warm disk surface of $f_{H2}$~>=~0.47 and derive amolecular abundance ratio of CO/H2~=~1.6$^{+4.7}_{-1.3}$~x~10$^{-4}$, bothconsistent with canonical interstellar dense cloud values.

CO/H2 Abundance Ratio ~ 10^{-4} in a Protoplanetary Disk