We propose to use the ISO Long Wavelength Spectrometer (LWS)
 to measure the line intensities of several high-J 12CO, 13CO
 and C18O rotational lines in different massive star formation
 regions.
 Theoretical calculations for UV-heated high density molecular
 clouds result in a large amount of hot CO. Therefore, even
 the high rotational lines are excited and should be detectable.

 A detection of these lines results in detailed information
 on the still unknown excitation conditions for these high-J
 lines. As especially the intensities of the high-J CO lines
 depend sensitively on the hydrogen particle density and UV-field
 strength, these lines are an excellent tool to determine the
 physical conditions. The chemical composition and isotopic CO
 abundance ratios will be estimated using the isotopic CO line
 intensities.