The pressure of the interstellar medium is a parameter of
 fundamental importance in steady-state models of a multiphase
 interstellar medium. However, this quantity is exceedingly
 difficult to measure. ISO provides a unique opportunity to
 improve this situation with measurements of the 158 micron CII
 line. This line is an excellent pressure probe for the atomic
 medium. We thus propose to use LWS to obtain a 1-dimensional strip
 map over the entire HI disk of the Local Group Galaxy M33 in
 the 158 micron CII line and the 122 micron NII line. We will
 thus derive the pressure in the cold medium as a function of
 galactocentric radius, but more importantly, be able to measure
 the differences in pressure between the spiral arms and star
 formation regions and the lower density regions in between.
 From the line ratios we will be able to determine the relative
 proportions of warm ionized medium and neutral medium and the
 variation with position within the galaxy. The measurements will
 be of fundamental importance for our understanding of the energy
 balance in the interstellar medium and M33 is the only galaxy
 where there the LWS spatial resolution is sufficient to resolve the
 spiral structure. We have sensitive WSRT synthesis imaging in HI
 of the entire galaxy which will be important for interpeting the
 CII emission line data.