NUMEROUS observations have been recorded which relate to whether an organic compound is dissolved, not dissolved, or decomposed by sulphuric acid : and if dissolved, whether or not it is recoverable by dilution ; and whether or not the solution is coloured. Such observations have been of value in qualitative organic analysis, but for many years they remained empirical. The study of the nature of solutions of organic substances in sulphuric acid was initiated by Hantzsch,l who made a cryoscopic investigation of a wide range of compounds in l OOyo sulphuric acid. The interest which this work aroused has in recent years considerably intensified.Before reviewing in detail the behaviour of various groups of organic compounds, we shall consider why this subject has been so extensively studied.(1) The wide variety of ways in which organic substances behave when treated with sulphuric acid itself provokes interest. This may be illustrated by considering the effect, at room temperature, of sulphuric acid on the following compounds : benzene, glucose, benzyl alcohol, methyl benzoate, ethanol, methyl mesitoate, and dicinnaniylideneacetone. Benzene does not dissolve, glucose is charred, and benzyl alcohol instantly forms an insoluble pink resin. The remaining substances dissolve, but if the solutions are immediately diluted with ice-water, these further results are found : the methyl benzoate is precipitated unchanged, the ethanol is found to have been esterified, the methyl mesitoate hydrolysed, and ths dicinnamylideneacet one sulphonat ed.(2) By the use of sulphuric acid we can observe the behaviour of organic molecules in an environment of extreme acidity, since sulphuric acid is the most strongly acidic solvent yet studied. Hence, it is not surprising that many substances function as bases in sulphuric acid which from their behaviour in other solvents are not generally considered to have basic properties. These compounds, which ionisc according to the equation X + H,SO, = XHEf + €€SO,-, include, for example, many ketones, carboxylic acids, and even some aromatic hydrocarbons. It is possible to study the relative basicities of such extremely weak bases by accurate cryoscopic and spectrometric measurements. Certain compounds ionise in a more complex fashion, to give stable cations that are of great interest.Examples are the nitroniuni ion NO,+ from ethyl nitrate, the mesitoyl ion (CH,),C,H,*CO+ from mesitoic acid, and the triphenylcarbonium ion Ph,C+