The sulfonation of bromobenzene with sulfur trioxide in liquid sulfur dioxide at -12 to -13° C. was studied to determine its relative reactivity. The relative rate constant ratio, kbenzene/kbromobenzene, was found to be 28.6 in a competitive reaction between benzene and bromobenzene. The isomer distribution was found to be 0.15 ± 0.02% for the meta, 0.91 ± 0.04% for the ortho and 98.93 ± 0.14% for the para isomers. The partial rate factors were calculated to be pf= 0.208, of= 0.0096 and mf = 0.00018. The relative rate data exhibited a distinctive behaviour previously noted in related systems, possibly due to secondary reactions. Data, generated artiticially, which showed similar behaviour, indicate that possibly the primary and secondary reactions both produce the final products in competition with each other at significantly different selectivities.