The proportion of nicotine in the vapour phase of different dilutions of mainstream tobacco smoke has been investigated. The percentages of the total available nicotine initially in the vapour phase were (0.5 +/- 0.3)% and (0.7 +/- 0.4)% when the averaged initial particle number concentrations were (3.8 +/- 0.6) x 10(7) and (1.2 +/- 0.1) x 10(9) particles per cm(3), respectively. The maximum percentages of the total nicotine that entered the vapour phase via evaporation from the particles decreased from (21 +/- 5)% to (4 +/- 3)% at the above respective concentrations. Results implied that there is a tendency towards equilibrium and that further dilution is required to re-establish evaporation. Further experiments, in which a 35 cm(3) puff of mainstream tobacco smoke was drawn from a chamber, found that the amount of nicotine evaporating from the particles increased linearly as the initial particle number concentration of each puff increased. However, as the initial concentration approached (2 +/- 1) x 10(7) particles per cm(3), further increases were found to have a limited affect upon the amount of nicotine evaporating from the mainstream tobacco smoke particles.