A correction technique for powder diffraction data recorded in transmission geometry is proposed. It considers the influence of sample inhomogeneities on the absorption of the scattered X-rays. The correction procedure is given for three typical types of samples : (i) homogeneous samples with constant thickness and small absorption contrast, (ii) samples with locally fluctuating sample thickness, and (iii) samples with non-negligible absorption contrast. The given expressions were tested by applying them to Rietveld analysis of powder diffraction data of YBa 2 Cu 3 O 7-δ specimens.