Abstract: The mass spectrometric behavior of 23 arylmercuric chlorides has been studied. The majority of compounds give intense peaks in the parent ion region. The fragmentation behavior observed for the molecular ions may be expressed as follows: In was found that in every case studied formation of D is favored over cleavage of the mercuryhalogen bond which produced ion E. This result obtained in our investigation is consistent with that observed by other authors, and then may be considered as a generalized rule for the fragmentation of organomercurials.It has been shown that the electronic character of the substituents has pronounced effect on the stability and the abundance of the fragment ions. With respect to most of meta and para substituents the parameter, log(Z/Z_H), representing the substituent effect on the rate of formation of D ions, shows a good linear correlation with the Hammett-Brown constants σ⁺. In contrast with p- and m-ethoxycarbonyl-phenylmercuric chlorides, o-ethoxy-carbonyl-phenylmercuric chloride undergoes the McLafferty rearrangement more difficultly and this feature is probably associated with the intramolecular coordination which exists in the molecule of o-ethoxycarbonyl-phenylmercuric chloride.