  The physical properties of molecular gas can be deduced from the analysis of
line intensity ratios, most commonly of the CO isotopomers 12CO and 13CO.
Barred galaxies, especially those with central starbursts, provide an excellent
laboratory to study the changing properties of the gas as it approaches the
center. Four galaxies (UGC2855, UGC2866, NGC7479 and NGC4123) are presented as
examples of the results that can be obtained. Special attention is given to the
case of (abnormally) high 12CO/13CO R_12/13 line ratios (exceeding 20), which
may indicate unusual gas properties. Qualitative scenarios for the structure of
the ISM explaining a range of values of R_12/13 are discussed.
