The composition and phase behavior of some lipid classes and mixtures of thylakoid polar lipids were measured to investigate their role as determinants of the temperature of the transition associated with chilling injury. For Neriurn oleander L., a plant which acclimates to growth temperature, a mixture of the phosphatidylglycerol (PG) and sulfoquinovosyldiacylglycerol (SQDG) showed transition temperatures of 22 ~ and 10~ for plants grown at 45 ~ and 20 ~ C, respectively. This difference was similar to the 9 Celsius degrees differential in the transition of the polar lipids and indicated that the PG and-or the PG-SQDG mixture could be the major determinants of the transition temperature. Reconstitution of the PG-SQDG mixture from 20~ oleander with the galactolipids from 45~ plants, however, reduced the transition temperature by only 4 Celsius degrees. This indicates that some, low-melting-point lipids, which are structurally capable of forming a co-gel with the high-melting-point lipids, also play a role in determining the temperature of the transition and that the composition of these low-melting-point lipids also changes with growth temperature. More specific information on the role of PG was obtained using polar lipids from Cucumis sativus L., a chilling-sensitive plant. For this material the transition in the polar lipids was reduced from 9 ~ to 5 ~ and 4~ when the transition of the PG was reduced from 32 ~ to 25 ~ and 22 ~ C. This was accomplished by reducing the proportion of disaturated molecular species in PG from 78 to 56 and 44 mol% by the addition of a fraction of the PG enriched in unsaturated molecular species. The data indicate that the transition temperature of the polar lipids of cucumber would be reduced to below 0 ~ C, typical of a chillinginsensitive plant, when the transition temperature of PG was reduced to 15 ~ C and this would occur at 21 tool% of disaturated molecular species.