  We present the results from ASCA observations of the centrally enhanced
supernova remnant 3C 391 (G31.9+0.0). We use the ASCA SIS data to carry out an
investigation of the spatial and spectral properties of the X-ray emission from
this remnant. The collisional equilibrium ionization and non-equilibrium
ionization spectral fits indicate that the hot gas within the remnant has
basically reached ionization equilibrium. The variation of the hydrogen column
density across the remnant is in agreement with the presence of a molecular
cloud to the northwest. The comparisons of hydrogen column and X-ray hardness
between the NW and SE portions of the remnant support a scenario in which the
SNR has broken out of a dense region into an adjacent region of lower density.
The mean density within the SNR is observed to be much lower than the immediate
ambient cloud density. This and the centrally brightened X-ray morphology can
be explained either by the evaporation of engulfed cloudlets or by a radiative
stage of evolution for the remnant.
