Abstract In the present paper, a direct observation of particle swelling behaviour and bubbling phenomena of individual pf size particles from two Australian coals during pyrolysis has been conducted using a single particle reactor. Density separated coal samples prepared using the sink–float method were used in the experiments. The bubble growth and ruptures during the plastic stage were monitored using a CCD video camera equipped with a long distance microscopic lens. Intensive bubbling phenomena were observed, which contributed to the swelling mechanism for coal particles while undergoing softening and swelling. Particle shrinkage during plastic stage due to bubble rupture was also observed. Significant distinctions of the swelling behaviour and bubbling phenomena among density fractions from the same coal were observed. Transient swelling ratios decreased markedly with increasing coal density. The morphology of the solid char residues changed significantly among different density fraction samples. This study extends current understanding of the heterogeneous nature of coal for char formation.