Summary The gas phase isomerisation of o-dichlorobenzene (odCB) was studied over protonic zeolites: HZSM5, HMOR, HMAZ, HOFF, HBETA and a pillared clay HPILC. All the catalysts deactivate. The deactivation rate is the highest when dry air is used as carrier gas, and the lowest with nitrogen containing water. The rate law of odCB isomerisation obeys the Langmuir-Hinshelwood formalism: ro = k λP/(1+ λP). The adsorption step is correlated with a Hammett equation log K = log ko + σ+p, and the negative value of p reflects the development of a positive charge in the transition state. The rate of the surface reaction, 1,2-shift of halogen species X, is better correlated with an homolytic C-X cleavage than with an heterolytic cleavage. The initial activity of the fresh protonic zeolite depends on both zeolite structure and Al content. The conversion of odCB gives predominantly isomerisation. The bimolecular transhalogenation of odCB to chlorobenzene and trichlorobenzenes remains lower than in homogeneous phase.