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.