The applicability of the often employed Faxen correction function for viscosity measurements of silicate melts at high temperatures with the falling sphere method has been investigated. Dynamic neutron radiography was used for the in situ and real time observation of spheres falling axially in a melt contained in crucibles with discontinuously changing, diameters. Ratios of sphere diameters, d, to crucible diameters, D, were 0.06-0.25, which covers most of the relevant range in which falling sphere experiments are carried out. Experiments were conducted at temperatures of 1045-1110 degreesC resulting in melt viscosities of about eta = 10(2) Pa s. Flow around spheres was fully laminar as maximum Re numbers were 10(-3). The widely used Faxen correction function was found to significantly over-compensate the retarding effect of the crucible walls on the sphere velocity. (C) 2003 Elsevier Science B.V. All rights reserved.