Actinide samples were characterized in an interlaboratory comparison between IRMM and NIST, including alpha-particle counting at defined low solid angle and counting in a 2π proportional gas counter. For this comparison, nine 233UF4 samples with high uniformity in the layer thickness were prepared at IRMM by deposition under vacuum. Polished silicon wafers were used as source substrates, and these were rotated during the deposition using a planetary rotation system. The estimated uncertainties for the defined low solid-angle methods were about 0.1% at both NIST and IRMM. The agreement of reported α-particle emission rates in the energy range 2.5-5.09 MeV was better than or equal to 0.02% for the defined solid-angle methods. When comparing total α-particle emission rates over the larger energy range 0-9 MeV (which includes all emissions from the daughter nuclides and the impurities), the agreement of the defined solid-angle methods was better than or equal to 0.05%. The 2π proportional gas counter results were about 0.75% higher than those of the defined solid-angle methods. A re-examination of the correction for scattering from the polished silicon wafer substrates was undertaken to explain this discrepancy. Preliminary results of the recalculated 2π proportional gas counter results are also given.All uncertainties in this paper correspond to one standard deviation.