This work describes a study which has been made of some of the physical aspects of the vapour-phase pyrolysis of the high-boiling ester, vinyl benzoate, the project being part of a wider study of the pyrolysis of ''Terylene" which has been pursued in these laboratories. The major part of the work involves a technique which is described whereby the rate of degradation of the involatile ester is followed empirically by measuring the rate of gas evolution from a system containing the vapourised ester together with a quantity of liquid reflux. Analysis of the pyrolysate gases is also applied to track the reaction. The primary findings of the study are the following; the complex degradation reaction is found to be mainly homogeneous; evidence for the absence of free-radical chain reactions is presented, and three individual primary decomposition routes are concluded to proceed by molecular mechanisms; catalytic effects in the presence of carbonaceous deposits, benzoic acid, and iodine are recorded. A short parallel study of the pyrolysis of vinyl acetate is also reported. The reaction is concluded to be homogeneous, and involving no free-radical chain processes.