The work described in this thesis represents an attack on certain problems of separation and identification which are presented by fish liver oils. A way has been sought for separating the mired highly-unsaturated acids which are present (together with numerous saturated and mono- olefinic acid ¿fa as glycerides in fish oils, without causing damage thereto by excessive heating or oxidation. There is described a method and technique of high vacuum-distillation (molecular distillation) which has giver separative results which surpass expectation. It is shown that the physical basis of evaporative distillation is such as to preclude separation of the unsaturated members of the fatty acid series (or their esters) which have the same chain length bub differ in the degree of unsaturation; it should however be possible to separate the groups of acids of the same chain-length. By the technique of molecular distillation described, the mixed unsaturated fatty esters from cod liver-oil, cod liver-halibut oil and Japanese sardine oil have been divided with very satisfactory precision into fractions consisting of G C 16 ’ 18 » ^20~> an^ (mixed) higher acids. The characteristic and most abundant highly unsaturated acid of the fish oils is shewn to be a "'22~'hexaene acid (^22^32^2^ ari(^ rt0^ ’the C^-pentaene a°id called Clupanodonlc acid as formerly supposed. Some progress, has been made in the determination of the constitution of this acid, and the possible formulae narrowed down to five. A means of distinction between the "natural” highly-unsaturated acids from fish oils and their heat-changed derivatives (the latter have usually previously been described as "natural" acids) is discussed in connexion with the refractivity/hydrogen value-relationship of the acids, and the claims which have been made in the literature respecting the isolation of pure individual acids are critically reviewed. A section of the thesis is devoted to the determination of the constitution of a highly unsaturated vegetable-oil acid viz punlcic acid from the seeds of Punlea Granatum . This acid is shown to be a genuine third geometrical isomeride of elaeostearic acid.