Abstract
Attention is given to the first detection of the interstellar 2(11)-2(12) K-doublet absorption line of H2C-13O. It is noted that the central radial velocities of the lines toward W33 and DR21 differ from those measured for the 1(10)-1(11) lines of H2C-13O. The differences found for the radial velocities of several H2CO lines are probably due to H2 density variations in these molecular clouds. Using only the higher K-doublets, an H2 density of 5000/cu cm and an H2C-12O/H2C-13O ratio of 25 + or - 3 are found, on the basis of the optical depth of these species in the 2(11)-2(12) line. The isotope ratio is 10, after correction for photon trapping.