It is widely thought that the intercalation of ionic species in solids is unaffected by non-ionic species in solution. But in the present work we report that, in some rare cases, low concentrations of non-ionic species in solution can actually catalyse the intercalation of ionic species in the solid state. Cyclic voltammetry, quartz crystal gravimetry, X-ray diffraction, and molecular modelling are used to confirm the existence of this remarkable effect. The origin of the effect is traced to the co-incorporation of the non-ionic species in the solid state, which causes the crystal structure to expand. The rapid incorporation of the ionic species can then occur. This discovery suggests that other solid state reactions might be catalysed in a similar way. The solid state species which are shown to exhibit the catalytic effect include three rhenium carbonyl complexes, namely trans-Re(Br)(CO)(dppe)(2), trans-Re(Br)(CO)(dppe)(dppm) (dppe=Ph2P(CH2)(2)PPh2; dppm=Ph2PCH2PPh2), and trans-Re(Br)(CO)(dpbz)(dppm) (dpbz=o-(Ph2P)(2)(C6H4); a chromium carbonyl complex, namely trans-Cr(CO)(2)(dppe)(2); the high-stability electron shuttle tetraphenylviologen dichloride; and the propeller-shaped redox reagent decaphenylferrocene. The solvents which can act as catalysts include acetonitrile, acetone, methanol and ethanol. (C) 2003 Elsevier Science B.V. All rights reserved.