An ammonium analogue of the main component of the anti-arthritic drug myochrysine, disodium thiomalato-S-arate(I) polymer, is examined by both positive- and negative-ion electrospray ionization (ESI) mass spectrometry; the principal peaks are those of a tetrameric species; subsequent examination of a drug sample by the same technique showed that the principal peaks were also those of a tetrameric species.