  ISO spectral observations of the supernova remnant RCW103 are presented. This
object is the prototype of relatively young remnants with fast shocks
interacting with dense interstellar medium. The spectrum is dominated by
prominent lines of [NeII], [SiII], [FeII] and other low excitation species
which provide, for the first time, a simple and reliable estimate of the gas
abundances of refractory (Si, Fe, Ni) and non-refractory (Ne, P, S, Cl, Ar)
species. Apart from nickel, all the derived abundances are close to solar,
confirming that the shock has destroyed all dust grains. Like the optical
nickel lines, [NiII] 6.64micron yields Ni abundances a factor ~10 solar which
we propose results from a large underestimation of the computed Ni+ collision
strengths. The observed intensities and velocity widths of ionic lines are
compatible with emission from the post-shock region alone with only a very
small (if any) contribution from the photoionized precursor. This result does
not agree with shock models which predict that the precursor should emit
powerful line emission, especially from highly ionized species. The possible
consequence of this on the modelling of Seyfert spectra is briefly discussed.
