The interrelated effects of temperature and fluorine concentration on fluorine-olefin reactions have been investigated with some perhaloolefins. Reactions were carried out at low temperatures in a new apparatus which was equipped with a novel high speed nickel gauze stirrer, gas disperser. Reproducible results were obtained with weight recoveries of 95% or better. With CF3CF=CFCF3, increasing the effective fluorine concentration greatly reduced the formation of dimer addition product relative to simple addition product. Lowering the reaction temperature had a similar effect due to reduction in the rate of the fluorine-olefin free radical initiation reaction. Related experiments with CClF=CClF, CF2=CCl2, and CF2=CClF, and a mixture of CF3CF=CFCF3 and CClF=CClF, are also reported. With CF2=CClF the fluorine-olefin reaction was diffusion rate-controlled at temperatures as low as -150°. The observed effects of reaction conditions on product composition, are shown to be consistent with the general free radical mechanism previously presented for fluorine-olefin reactions and to provide a guide for the choice of synthesis conditions. The liquid phase fluorination procedure described is considered suitable for general use.