Advanced glazing materials may be thick, scattering, and inhomogeneous, and this poses problems for conventional absorptance measurement techniques. This paper describes a technique for accurately measuring the spectral absorptance of such materials. The measurement technique, in which the sample is mounted at the centre of an integrating sphere, was analysed and the errors that occur when applying the technique to large samples were analysed. The technique was used to measure absorptance of several materials, and the results are presented. For heterogeneous materials it is necessary to irradiate a relatively large area to get a representative sample. It was found that under certain conditions (depending on the sample absorptance), sample sizes as large as 30% of the sphere diameter can be used without incurring a prohibitively large error. This paper examines the effect of the finite absorptance of the rod supporting the sample, showing that it can be important unless certain precautions are taken.