This thesis describes a series of studies of the physical behaviour of commercially important stabilizing additives in polyolefins. A series of nickelthiocarbamates have been synthesised and their solubilities measured as a function of temperature in linear hydrocarbon solvents, low density polyethylene and isotactic polypropylene. The results are compared with predictions from a regular solution model. The loss of three benzophenone UV-stabilizers from supersaturated solution has been investigated with the aim of determining whether the loss is a diffusion controlled blooming or involves phase separation. Our data shows this phenomenon is a diffusion controlled blooming process. The volatilities of these same UV-stabilizers have been measured as a function of temperature. The solubility of a typical hindered amine (Tinuvin 770) has been investigated in hydrocarbon solvents and in polypropylene by infrared methods. Low solubility values were obtained in all cases. The diffusion of Tinuvin 770 in polypropylene has been monitored by an infrared method developed for the purpose and the diffusion coefficient estimated over a range of temperatures. Litp.rature data on solubility of additives together with our own data are critically reviewed and the effect of structure on solubility is analysed. Data on the benzophenone additives allows prediction of loss times and mechanisms to be made for the first time.