A study is reported of the determination of C14 in C14-labeled substances by counting, with a proportional gas-flow counter, the radioactive disintegrations arising from a material that is dissolved in a suitable solvent. Formamide is recommended for assay of carbohydrates and other materials with which it is a compatible solvent. Ethylene glycol is suitable for assay of highly alkaline substances, and, in conjunction with sequestering agents, for assay of alkaline earth carbonates. Other useful solvents are dimethyl formamide and 90% phosphoric acid. The counting efficiency in different solvents was inversely proportional to the density of the solution and highly reproducible. The study shows that solutions of C14 -labeled materials can be counted easily and precisely with a proportional counter.