Interdigitated array (IDA) electrodes were used to study the electrochemical behaviour of the soluble redox protein cytochrome c. The IDA electrodes consisted of 70 pairs of thin-film gold microbands, each of it with 2.9 mu m band width and 1.1 mu m interelectrode spacing. The surface of the gold electrodes was modified by 4,4’-dithiodipyridine and N-acetyl cystein in order to promote the electron transfer between cytochrome c and the electrodes. The close spacing in the interdigital arrangement allows generation/collection experiments with the oxidized and reduced form of cytochrome c. Applying this technique the diffusion coefficient of cytochrome c in buffered aqueous solution was determined to be 8.37 x 10(-7) cm(2) s(-1).