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(Abridged) We present an analysis of X-ray observations made of the Galacticsupernova remnants (SNRs) HB21 (G89.0+4.7) and CTB 1 (G116.9+0.2), twowell-known mixed-morphology (MM) SNRs. We find a marked contrast between theX-ray properties of these SNRs: for HB21, the extracted ASCA spectra of thenorthwest and southeast regions of the X-ray emitting plasma can be fit with asingle thermal model with marginally enhanced silicon and sulfur abundances.For both of these regions, the derived column density and temperature areN_H~0.3x10^22 cm^-2 and kT~0.7 keV, respectively. No significant spatialdifferences in temperature or elemental abundances between the two regions aredetected and the X-ray-emitting plasma in both regions is close to ionizationequilibrium. Our Chandra spectral analysis of CTB 1 reveals that this source islikely an oxygen-rich SNR with enhanced abundances of oxygen and neon. Theextracted ASCA spectra for the southwestern and northeastern regions of CTB 1cannot be fit with a single thermal component. Based on our fits to thesespectra, we derive a column density N_H~0.6x10^22 cm^-2 and a temperature forthe soft thermal component of kT_soft~0.28 keV. The hard emission from thesouthwest may be modeled with either a thermal component (kT_hard~3 keV) or bya power law component (Gamma~2-3) while the hard emission from the northeastmay be modeled with a power law component (Gamma~1.4). We have also extractedASCA GIS spectra of the discrete X-ray source 1WGA J0001.4+6229 which is seenin projection toward CTB 1. These spectra are best fit using a power-law modelwith a photon index Gamma=2.2^{+0.5}_{-1.2} which is typical for featurelesspower-law continua produced by rotation-powered pulsars. This source may be aneutron star associated with CTB 1.

Mixed-Morphology Supernova Remnants in X-rays: Isothermal Plasma in HB21 and Probable Oxygen-Rich Ejecta in CTB 1