The pyrolysis of ethane was performed in tubular reactors at temperatures from 1110 to 1220 K, at pressures from 40 to 80 kPA and at residence times from 1.4 to 58 s. An aerosol was observed by laser attenuation. The aerosol is believed to be an intermediate between gaseous polynuclear aromatic hydrocarbons and soot. Formation of the aerosol led to an orange glow. Radial concentration gradients of the aerosol were measured using video analysis. Still images were processed with a computer code to determine the change in intensity as a function of position and time. The results were consistent with the theory of non-steady diffusion in a cylinder. The diffusion coefficients and average particle diameters of the droplets were determined. Samples of the aerosol were colected using hot quartz probes and were morphologically characterized using electron microscopy. The projected area, the perimeter, the Feret's diameter and the circularity of the particles were determined with image processing software. The results indicated the presence of a polydisperse aerosol with particle sizes between 4 and 360 nm.