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Abstract.We have studied the ratio of 13 are given. These ratios were found to vary from cloud to cloud so that they are larger in B 335 than in B 133 and L 466. These variations are thought to primarily arise from variations of the column density ratio of CO and its isotopes to H 2 . The alternative explanation in terms of the N(H 2 )/A J ratio being larger in starforming regions (B 335) than in more quiescent regions lacking star formation (B 133 and L 466) appears less likely. We also discuss the evidence for N(CO)/A J variations on the basis of previous observations in star forming and non-star forming dark clouds.

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