The pyrolysis of ethane was performed in tubular reactors at temperatures from 1110 K to 1220 K, at pressures from 40 kPa to 80 kPa and at residence times from 1.4 s to 58 s. The formation of a fog was monitored using laser extinction. The boundaries of the regime of time, temperature and pressure in which a condensed phase was formed were delineated. Five simple analytical models were derived in order to analyze the kinetics of formation of this phase. The parameters in the models were fitted to the experimental data and were compared to the predictions of the kinetic theory. The most successful model involved a steady formation of precursors until a critical pressure was reached. Nucleation then occurred rapidly, followed by a steady growth of the volume of the particles. The temperature dependence of the inverse of the incubation period was determined to be In[(theta/s)(-1))] = -(200 +/- 30) kJ mol(-1)/RT + (20 +/- 4). The rate of particle growth was proportional to the square of the reactant pressure and followed the following Arrhenius expression: In[omega/s(-1)] = -(420 +/- 40) kJ mol(-1)/RT + (39 +/- 4). According to this model the heat of vaporization of the droplets was (210 +/- 50) kJ mol(-1). This was consistent with condensation of polynuclear aromatic hydrocarbons having molecular weights of about (460 +/- 110) g mol(-1). At longer residence times the attenuation of the laser beam reached a plateau. This was interpreted in terms of a decline in the rate of fog formation or in terms of the removal of droplets by deposition on the reactor surface.