The temperature coefficient of e.m.f. of a cell consisting of a saturated calomel electrode and a redox electrode of K4Fe(CN)6 and K3Fe(CN)6 has been determined by e.m.f. measurements at a series of equimolal concentrations of the above salts and at temperatures of 20, 25, 30, 35 °C. The results for this calomel–redox cell are expressed by the equation ECR0 = 0.1200 − 0.00214(t − 25), or for the redox electrode against a hydrogen gas electrode, EHR0 = 0.3644 − 0.00280(t − 25), with E in V and t in °C. Analogous results are available in the literature for other such pairs of complex ions; an examination of these data leads to the hypothesis that the following trichelated pairs: Fe dip32+/3+, Fe phen32+/3+, Ru dip32+/3+, are respectively nearly isentropic.