Kinetics of the iodination of a number of substituted benzenes and thiophenes by iodine and nitric acid in acetic acid solution have been studied. The reaction is catalysed by dinitrogen tetroxide and hydrogen ions and evidence is presented to show that the iodinating species is protonated nitryl iodide, which reacts in a slow step with the substrate. Acidity function data for nitric acid in 10% aqueous acetic acid are presented and the charge-transfer complex formation between iodine and m-xylene in acetic acid has been studied. The conversion of 4-iodoanisole into 2-iodo-4-nitroanisole by nitric acid, has been shown to occur by rapid nitrodeiodination and iodination to give 2,4-di-iodoanisole and 4-nitroanisole; gradual nitrodeiodination of the former compound at the 4-position with the production of free iodine, and then very slow iodination of the latter compound at the 2-position, Nitrous acid and hydrogen ions act as catalysts and the preliminary step is thought to be nitrosodeiodination. The ortho:para ratio for the methoxy-group in nitrosodeiodination is very low. The ipso factors of Perrin and Skinner are discussed in the light of this and other work. Transient intermediates observed in the nitrodeiodination reaction have been investigated using stopped-flow techniques and a water soluble substrate. These species are tentatively suggested to be o-complexes. The anomalously high rate of nitration of iodobenzene has been investigated and it has been found that this phenomenon is not due to a nitrodeiodination reaction, as had been suggested previously. Work on the nitration of ioclomesitylone has been investigated and it has been found that nitrosodeiodination is significant in this reaction, which is contrary to the findings of the previous workers. A computer programme has been written to assist calculations on data obtained from radio-isotopic dilution experiments where two products are formed simultaneously from a single substrate. Another computer programme has been written to assist in the calculation of rate constants for reactions of the first and half order.