  The giant molecular cloud complex, Sgr B2, was observed with the ISO Long
Wavelength Spectrometer over the range 47-197 microns. The ground state
rotational transitions of OH lie in the survey range and we have analysed
results from the 16OH, 17OH and 18OH isotopomers. Absorption in these lines due
to clouds along the line of sight towards Sgr B2 has allowed us to compare the
isotopomer abundance at different galactocentric radii. This is an excellent
test of previous results for the isotope ratios which were measured at radio
wavelengths and generally required a double ratio with 12C/13C.
