Theoretical modelling suggests that all molecular clouds contain significant column densities of cold atomic hydrogen. We have found that 21 cm H I self-absorption features toward GRS molecular clouds caused by this cold H I allow us to resolve the long-standing near/far kinematic distance ambiguity. Because we can measure the distances to clouds and their embedded infrared young stellar objects and star clusters, we can establish their masses, sizes, distributions, and luminosities.

We have also studied infrared dark clouds revealed by the Mid--Course Space Experiment (MSX) infrared survey. These clouds are easily detected in ¹³CO emission. We find that many are in fact cores of larger giant molecular clouds. Because we can establish their kinematic distances, we can deduce their masses, sizes, and distributions. Their masses (∼few thousand solar masses) and sizes (∼few pc) suggest that these are the initial condensations that will ultimately become OB clusters or associations. Their radial Galactic distribution peaks in the 5 kpc ring.