Milligram amounts of bases of various classes, viz., compounds that contain primary, secondary or tertiary amino groups, quaternary ammonium compounds and simple ammonium salts, have been determined by utilising their reaction with a known and excess amount of sodium tetraphenylboron. The excess of the boron compound has been determined by direct injection enthalpimetry by using potassium chloride. The results were all within 1·5 per cent. of the theoretical value. The standard deviation of the method applied to the determination of aniline (38 mg) was 0·27 mg.