A method for the measurement of absolute 238U capture rates is described. The principle is based upon the fact that 243Am and 239U decay via 239Np into 239Pu. The α disintegration rate of 243Am is determined absolutely by low geometry α-particle counting. From this, the absolute 239Np disintegration rate follows directly, which is measured by the 106-keV γ-x-ray coincidence technique. Therefore, the explicit knowledge of detector efficiency and decay-scheme data is not necessary, and also radiochemical separation is avoided. The absolute 238U capture rate can then be obtained from the known 239Np activity. The γ-x-ray self-attenuation within thick uranium foils is treated quantitatively and an optimum foil thickness is derived. Special attention has been given to the reduction and correction of background due to fission-product γ activity. A detailed error analysis shows an overall accuracy of about 1% for the absolute 239Np disintegration rate including γ attenuation effects within the foils. To ascertain the reliability of low-geometry α calibration, comparison was made with the α-γ coincidence method. Agreement was found within the error limits of 1%.