Soot formation during isothermal pyrolysis of naphthalene, anthracene and pyrene was investigated. The particle number density of soot aerosol formed was demonstrated to depend linearly on hydrocarbon concentration. The equations to calculate the particle number density and the soot surface area were obtained. Particle number densities of the soot formed during pyrolysis of polycyclic aromatic hydrocarbons were demonstrated to be an order of magnitude higher and the apparent activation energy of soot aerosol formation is two times less than the parameters obtained during pyrolysis of benzene or acetylene. Sooting tendency of the hydrocarbons investigated as relative to methane can be arranged al 1623 K as follows: methane: ethylene: acetylene: diacetylene: benzene: toluene: p-xylene: naphthalene: anthracene: pyrene: 1: 4: 7.6: 50: 7.4: 5.5: 4: 112: 91: 74.