In connection with fast neutron (n, α) cross-section measurements, a novel boron analysis method has been developed. The boron concentration is inferred from the mass spectrometrically determined number of helium atoms produced in the thermal and epithermal 10 B (n, α) reaction. The relation between helium amount and boron concentration is given, including corrections for self-shielding effects and background levels. Direct and diffusion losses of helium are calculated and losses due to gettering, adsorption, and HF-ionization in the release stage are discussed. A series of boron determination is described and the results are compared with those obtained by other methods, showing excellent agreement. The lower limit of boron concentration, which can be measured, varies with the type of sample. In, e.g., steel, concentrations below 10−5% boron in samples of 0.1 to 1 g may be determined.