The heat associated with the formation of 1 mole of sec.-hexyl hydrogen sulphate from 79.57 % w/w aqueous sulphuric acid and 1-hexene at 0" C has been estimated at 32.3 rt 024 kcal, employing an ice calorimetric technique. The results indicated that the subsequent hydrolysis of the sec.-hexyl sulphate to the alcoxonium ion was athermal to within f 0.6 kcal. These determinations have also thrown light upon the nature of the rate-determining process in this heterogeneous sulphation reaction.It is not possible to deduce theoretically the heat associated with the formation of sec.-alkyl hydrogen sulphates from a-olefins and aqueous sulphuric acid because first, there is no reliable information on the nature of the ions involved in the reaction and secondly, few relevant thermodynamic data are known for this class of compounds. Accordingly it was decided to measure this heat calorimetrically.This measurement is difficult for the following reason. The sec.-alkyl hydrogen sulphates, which are formed by the rate-determining addition of a proton to the a-olefin followed by the rapid addition of a hydrogen sulphate anion, can undergo a reversible hydrolysis via the carbonium ion to the corresponding alcoxonium ion.1 The reaction scheme may be represented as slow (fH+)

The initially heterogeneous reactions between 75 to 85% w/w aqueous H2SO4 and hexene‐1, heptene‐1, octene‐1, and decene‐1, have been investigated as a necessary preliminary to the estimation of the heat associated with the production of the monoalkyl sulphates. The effects of different proportions of acid and olefin, the aqueous concentration of the acid, the temperature and the speed of stirring, were examined. At room temperatures, the reaction products isolated with a high ratio of 80% w/w acid to olefin, were substantially the secondary monoalkyl sulphates and their corresponding alcohols. Under these conditions the formation of the dialkyl sulphates and polymers was avoided. It was found possible to represent approximately the results in terms of a two‐stage consecutive reaction. RCH : CH2 → RCH(O·SO3H)‐CH3 → RCH(OH2+)·CH3 The reactions were shown in each case to be of the second order depending upon some function of the concentration of the aqueous H2SO4. The observed kinetics were shown to be in agreement with the formation of a carbonium‐ion intermediate.