We present results of X-ray observations of a sample of 15 clusters selected via their imprint on the cosmic microwave background (CMB) from the thermal Sunyaev-Zel’dovich (SZ) effect. These clusters are a subset of the first SZ-selected cluster catalog, obtained from observations of 178 deg of sky surveyed by the South Pole Telescope. Using X-ray observations with Chandra and XMMNewton, we estimate the temperature, TX , and mass, Mg, of the intracluster medium (ICM) within r500 for each cluster. From these, we calculate YX =Mg TX and estimate the total cluster mass using a M500 − YX scaling relation measured from previous X-ray studies. The integrated Comptonization, YSZ , is derived from the SZ measurements, using additional information from the X-ray measured gas density profiles and a universal temperature profile. We calculate scaling relations between the X-ray and SZ observables, and find results generally consistent with other measurements and the expectations from simple self-similar behavior. Specifically, we fit a YSZ − YX relation and find a normalization of 0.82± 0.07, marginally consistent with the predicted ratio of YSZ/YX = 0.91± 0.01 that would be expected from the density and temperature models used in this work. Using the YX derived mass estimates, we fit a YSZ −M500 relation and find a slope consistent with the selfsimilar expectation of YSZ ∝M 5/3 with a normalization consistent with predictions from other X-ray studies. We compare the X-ray mass estimates to previously published SZ mass estimates derived from cosmological simulations of the SPT survey. We find that the SZ mass estimates are lower by a factor of 0.89 ± 0.06, which is within the ∼15% systematic uncertainty quoted for the simulationbased SZ masses. Overall, the X-ray measurements confirm that the SZ-selected sample consists of very massive systems which exhibit general properties consistent with X-ray selected samples, even allowing for the broad redshift range (0.29 < z < 1.08) of the sample. Subject headings: galaxies: clusters — X-rays: galaxies: clusters Electronic address: kanderss@space.mit.edu 1 MIT Kavli Institute for Astrophysics and Space Research, Massachusetts Institute of Technology, 77 Massachusetts Avenue, Cambridge, MA 02139 2 Kavli Institute for Cosmological Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 3 Enrico Fermi Institute, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 4 Department of Physics and Astronomy, Cardiff University, CF24 3YB, UK 5 University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 6 Department of Astronomy, University of Illinois, 1002 West Green Street, Urbana, IL 61801 7 Department of Physics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 8 Harvard-Smithsonian Center for Astrophysics, 60 Garden Street, Cambridge, MA 02138 9 Department of Astronomy and Astrophysics, University of Chicago, 5640 South Ellis Avenue, Chicago, IL 60637 10 Department of Physics, McGill University, 3600 Rue University, Montreal, Quebec H3A 2T8, Canada 11 Department of Astronomy and Astrophysics, Pennsylvania State University, 525 Davey Lab, University Park, PA 16802 12 Department of Physics, University of California, Berkeley, CA 94720 13 Department of Astrophysical and Planetary Sciences and De