We have studied the ratio of carbon monoxide column density to the extinction A_J_ of background field stars in the direction of three globules: B133, B335, L466. The N(CO) to A_J_ ratios were found to vary from cloud to cloud so that they are larger in B335 than in B133 and L466. These variations are thought to primarily arise from variations of the ratio N(CO)/N(H_2_).