[en] Observations of the 6 cm absorption lines of H212CO and H213CO have yielded values for the ratio of [12C] to [13C] of 62 in W3, 55 in NGC 2024, and lower limits for this ratio that are close to terrestrial in Cas A and M17. This provides additional evidence for substantial variation of the [12C]/[13C] ratio in interstellar matter. We have also observed the H141β recombination line of hydrogen in these sources and found several new sources of formaldehyde absorption