The closest molecular cloud L1457 represents a unique target to study
 the interfaces of molecular, atomic, and ionized gas phases at very
 small size scales; due to its vicinity (65 pc) the highest spatial
 resolution possible can be achieved. Since molecular clouds show a very
 inhomogenous spatial structure at all size scales the gas phases are
 expected to be mixed in high as well as low density regions. We want to
 determine the ratio of mixing in dependence of the density and the
 temperature to investigate the physics and chemistry. For this reason
 we propose to observe strip-like maps in fine-structur transitions of
 OI and CII along a steep intensity gradient in the southern dense core
 of L1457 with the ISO LWS. Moreover, observing of the OI 63 mum and
 the CII 158 mum transitions allow a direct determination of the most
 important cooling processes of molecular clouds gas which is not
 possible with ground-based telescopes but only with ISO. Although the
 CII 158 mum transition has already been detected with COBE in diffuse
 clouds, the sensitivity and angular resolution obtained with ISO will
 give a detailed picture of the small scale distribution of CII over
 an extended region; the same region will be mapped in OI 63 mum, so in
 addition this would be the first detection of OI in a diffuse cloud.
 Observing the proposed region, we will manage to cover continuously,
 with respect to the angular resolution auf ISO, a region containing
 high as well as low density gas, and determine in this way the
 variation of the gas phases abundancy ratio with varying excitation
 conditions.