Unless the area of an inlaid disc electrode is sufficiently large, and/or the scan rate fast enough, the 'plateau' of a semiintegrated d.c. voltammogram or aperiodic component of an a.c. voltammogram has a slope. This phenomenon, which has its origin in non-planar diffusion at the edge of the disc, interferes with an otherwise efficient method of determining diffusion coefficients. Methods of circumventing this difficulty are presented and tested with simulated and experimental data. Crown Copyright (C) 2015 Published by Elsevier B.V. All rights reserved.