In this study, aerosol formation from the ozonolysis of a-pinene is examined using technique particle sizing method. Details of the mechanism for aerosol formation were interrogated by ozonising enal and enone derivatives of a-pinene that allowed competing channels of the reaction to be isolated. The experiments showed quite clear differences in the characteristics of the aerosol generated from the two derivatives. In particular, the results showed that ozonolysis of the enal derivative readily generated aerosol while ozonolysis of the eneone did not. The results are discussed in terms of the known chemistry of the systems.