Isokinetic relationships of the series of electrophilic substitution reactions of o-substi-tuted phenylmercurials are studied, and the results show, for the first time, that isokinetic correlation analysis can reveal the dominant factor in the system of the reactions. The analysis of SE2 of o-substituted phenylmercuric chlorides indicates that the steric effect of o-substituents is the predominant factor when I2 is used as electrophilic agent, however, the electronic effect of o-substituents would become the predominant factor when HCI is used as the electrophilic agent. In SE1 of o-substituted phenylmercuric chlorides, the results of the analysis display that the predominant factor is the field effects of o-substituents. The determination of predominant factors in those reactions would provide further scientific basis for those reaction mechanisms. Isokinetic relationships of the series of electrophilic substitution reactions of o-substi-tuted phenylmercurials are studied, and the results show, for the first time, that isokinetic correlation analysis can reveal the dominant factor in the system of the reactions. The analysis of SE2 of o-substituted phenylmercuric chlorides indicates that the steric effect of o-substituents is the predominant factor when I2 is used as an electrophilic agent, however, the electronic effect of o-substituents would become the predominant factor when HCI is used as the electrophilic agent. In SE1 of o-substituted phenylmercuric chlorides, the results of the analysis display that the predominant factor is the field effects of o-substituents. The determination of predominant factors in those reactions would provide further scientific basis for those reaction mechanisms.