Each isoplithio peak, obtained by chromatography of a pancreatic ribonuclease digest of RNA, contains up to 2 n+1 different compounds of general formula (purine nucleotide) n pyrimidine nucleotide. Methods have been worked out for the quantitative separation of respectively the mono-, di- and trinucleotide fractions by chromatography on DEAE-Sephadex columns. Complete resolution was achieved, except that rechromatography on paper was necessary to separate the positional isomers GpApCp and GpApUp from ApGpCp and ApGpUp, respectively. The quantitative distribution of two mononucleotides, four dinucleotides and eight trinucleotides in yeast RNA and in MS2 RNA is reported. The two types of RNA show certain similarities. The results of the MS2 RNA digests have been recalculated as absolute numbers of tracts per molecule and compared to a random distribution. There is a small excess of Cp and ApCp. Among the trinucleotides ApApCp is exceptionally abundant, whereas only half the expected amount of ApGpUp is found. This may mean that the former is a frequently occurring code word, and the latter only rarely or not at all.