It was shown that data from product analysis give valid rate factors for free radical aromatic substitution, not only removing current doubts but putting these rate factors on firmer footing than ever before. A method of analysis of biaryl mixtures was worked out that gives rate factors about ten times as accurate as previous methods have given and a method was devised for collecting reference samples of biaryls that elimates the need for lengthy synthesis. Rate factors were measured by this method for eight peroxidesubstrate systems, and it was found that these results are consistent with the operation of polar effects. It was found that, in certain cases, the esters reported from peroxide decomposition are not the simple substituted phenyl benzoates previously assumed. Data were obtained about the effect of oxygen and peroxide concentration on yield of benzoic acid that strongly suggest the operation of a cage effect in free radical substitution.