The sizes and mass spectra of large ((N) over bar =1900-13 700 molecules) cold (similar to3.1 K) H-2 clusters have been measured after scattering from CO molecules. Cluster-size measurements after between 2 and 8 collisions indicate that 7% of the H-2 molecules are evaporated. This loss agrees with calculations for the number of H-2 molecules evaporated by the heat released in the transition from an initial liquid state to a final solid state. Even though heterogeneous nucleation is initiated after only a few collisions with CO molecules, the mass spectra show that additional captured CO molecules coagulate to form large CO clusters with up to n=11 molecules, suggesting that the outer layer is sufficiently liquidlike to facilitate mobility of the CO molecules. Since the calculated H-2 cluster temperature (similar to3.1 K) is below the superfluid transition temperature predicted for pH(2) with density between 40% and 80% of the triple-point density, a shell-like region of low density near the cluster surface can be expected to be superfluid. (C) 2004 American Institute of Physics.