Partially carbonized samples from three coals were prepared using a Juranek furnace, and the resultant fractions corresponding to temperatures over the range 290-630 degrees C were characterized by quantitative solid-state C-13 NMR at low field using single-pulse excitation (SPE) and optical microscopy to determine and correlate the bulk chemical and microscopic transformations that occur during the thermoplastic stage of coking. As found previously for coals, chars, and pitch fractions, the carbon skeletal parameters derived from cross polarization experiments for the partially carbonized coals are significantly lower than those found by SPE. In the temperature range from ambient to 350 degrees C, there were no appreciable changes in the carbon structures. The softening and vesiculation processes (370-450 degrees C) observed by optical microscopy correspond to the loss of volatile aliphatic material and lead to a small increase in aromatic bridgehead carbon. During resolidification and formation of mosaic textures at temperatures above 460 degrees C, the aromaticity continues to rise with a vast increase in bridgehead carbon, due to dehydrogenation and condensation of aliphatic and aromatic structures. The number of rings in the aromatic clusters determined by NMR correlates with the size of the mosaic optical texture, with the two higher volatile coals having both the same mosaic size of 0.7 mu m and 19 rings per aromatic cluster at 600 degrees C, while the medium volatile coal has a mosaic size of 2.0 mu m with 26 rings per cluster.