It is found that at low temperatures the sulphonation and nitration of naphthalene, and the further sulphonation and nitration of its nitro, surphonic acid and nitrosulphonic acid derivatives, takes place in accordance with theories advanced in Parts I1(a)II1(b)and III1(b)of this aeries. Only at high temperatures and with particular compositions of the reaction medium (e.g whether in sulphuric acid or oleum of various strengths) does sulphonation appear anomalous, although such anomalies have been resolved by, the explanations advanced in the previous parts. The introduction into general use of the rules of Veselý and Jake2, as modified by resonance theory, in place of those of Amstrong and Wynne3, therefore appears to be justified