Althouth many C-aldehydes of azoles and azines, including those of 1,2,3-triazoles, are known and readily obtainable, free aldehydes derived from 1,2,4-triazole do not appear to have been isolated previously. The main aim of the work submitted in this thesis was the synthesis of such 1,2,4-triazole-C-aldehydes. Frerichs and Beckurts (26) isolated the phenylhydrazone of 1-phenyl-5-hydroxy-1,2,4-triazole-3-aldehyde from the complex mixture produced by reaction of phenylhydrazine with chloroacetylurethane or chloroacetylurea, but did not prepare the aldehyde itself. Parkes (58) prepared the 2,4,-dinitro-phenylhydrazone of 0 (or N)-acetyl-3-hydroxy-1,2,4-triazole-5-aldehyde. The crude free aldehyde was formed by ozonolysis of the corresponding 5-styryl compound, followed by treatment with Adams' catalyst but it could not be obtained pure. Apart from these cases, 1,2,4-triazole-C-aldehydes or their derivatives have not been noted in the literature. The chemistry of the 1,2,4-triazoles has recently been reviewed (66), but relatively little is known when compared with the chemistry of other N-heteroaromatic series. A preliminary study of the properties of the 1,2,4-triazoles suggested that the aldehydes might not be prepared as readily as in some other series. A brief summary is therefore given of the types of reaction which 1,2,4-triazoles might be expected to undergo, with reference to what is known of the electron distribution in the nucleus. This is compared with observed reactivity, or otherwise, in the few specific cases which have been described or studied. From this, limitations imposed by the intrinsic properties of the nucleus could be taken into account, to to some extent, in seeking the course of reaction most likely to yield the desired compounds. Unfortunately, few quantitative measurements are available. Several 1,2,4-triazole-aldehydes have been obtained and observations made of the properties of the 1,2,4-triazoles during the course of this work, do, in general agree with what was expected.