Molecular clusters of a wide variety of substances have been generated by'homogeneous nucleation in nozzle flow and studied by electron diffraction. Clusters were the order of 100 Á in diameter. The majority of the clusters were found to be liquidlike. Two examples could be produced as either liquid or crystalline clusters by adjusting flow conditions. Several others could be induced to organize into two or more different crystalline packings by controlling mole fraction, carrier gas, and/or stagnation pressure. Substances exhibiting largeranges of liquid existence in the bulk gave liquid clusters while those with small or null ranges gave solid aggregates. In intermediate cases the entropy of fusion, representing a measure of difficulty of molecular reorientation, helpedto sharpen the diagnostic rule of thumb regarding cluster form. Approximate computations of the temperatures of clusters growing in their supersaturated vapor during nozzle flow indicate that the condensing clusters are appreciably warmer than the gaseous medium and much warmer than the grown clusters exiting the nozzle. In a number of cases, the clusters appear to have undergone phase changes in the course of some microseconds of cooling in adiabatic flow after they were fully grown.