An accurate measurement of the 4438 keV γ-ray to total neutron ratio, R = S γ S n , for a commercially obtained 3 mCi 241 Am 9 Be (α, n) source is described. The total neutron output of the source had previously been measured absolutely by the National Physical Laboratory by a technique directly traceable to their absolutely calibrated MnSO 4 bath. The decay of the source from the date of calibration was calculated on the basis of the half-life of 241Am; this correction was confirmed using the Harwell oil moderated assembly of BF 3 counters. The absolute yield of 4438 keV γ-rays was determined using a 101 cm 3 hyperpure germanium detector which had been calibrated using a combination of radionuclide and thermal neutron capture γ-ray sources. Particular attention was paid to the estimation of a number of other correction factors including those for (1) the attenuation of γ-rays in the source, (2) the finite size of the γ-ray source on the γ-ray detection efficiency, (3) the attenuation of neutrons in the sample, (4) the production of neutrons via (α, n) reactions in impurities and (5) the production of neutrons via (n, 2n) and (γ, n) reactions in beryllium. The final result of R = 0.591 (± 2.6%), although consistent with values reported by independent workers, is approximately 6% higher than the previous best measurement. In common with these workers, the main source of uncertainty in this measurement is that in the γ-ray detection efficiency. An extensive series of theoretical calculations was undertaken in order to see whether the R-value measured was reasonable and also to judge to what extent it may be influenced by details of the source construction. It was found that R is relatively insensitive to either the choice of α-emitter or to the particulate size of the α-emitting material. Consequently, provided details of the source fabrication are known and appropriate corrections are applied, the use of the product RS n as a means of deducing the γ-ray yield from a neutron intensity calibrated 9Be(α, n) source is permissible.