When Pt(NO2)(4)(2-) is dissolved in methanesulphonic acid, it is shown that nitrite ions are displaced by water ligands and the extent of ligand substitution depends on temperature, length of the thermal treatment and the concentration of acid. Although the final nitrite ligand is difficult to displace, a solution containing Pt(NO2)(H2O)(3)(+) as the dominant species may be formed, and this solution (Pt concentration 6 g dm(-3)) provides the basis of a reasonable Pt plating bath. At a temperature of 343 K, good quality coatings can be obtained with current densities in the range 1-15 mA cm(-2) although the current efficiencies are low (10-15%) because reduction of the nitrite ion competes with Pt deposition on the freshly formed Pt surface.