The stability of tetrahedral and cubic platinum nanoparticles during the catalysis of the electron-transfer reaction between hexacyanoferrate (III) and thiosulfate ions in colloidal solution at room temperature was studied by using TEM and HRTEM. Before the reaction, the dominantly tetrahedral nanoparticles have a shape distribution of 55 +/- 4% regular tetrahedral, 22 +/- 2% distorted tetrahedral, and 23 +/- 2% spherical nanoparticles, and the dominantly cubic nanoparticles have an initial shape distribution of 56 4% regular cubes, 13 +/- 1% distorted cubes, and 31 +/- 3% truncated octahedral nanoparticles. The amount of tetrahedral nanoparticles decreases by 60 +/- 5% after the first cycle and by 62 +/- 4% after the second cycle of the reaction. In the case of cubic nanoparticles, the amount of cubic nanoparticles decreases by 39 +/- 5% after the first cycle and by 66 +/- 5% after the second cycle compared to before the reaction. After the first and second cycles of the reaction, there are a greater percentage of distorted tetrahedral and distorted cubic nanoparticles present. The rate of the dissolution of the surface Pt atoms is faster for the tetrahedral nanoparticles than for the cubic nanoparticles. This suggests that tetrahedral nanoparticles, with their sharp corners and edges, are more sensitive and more liable to shape changes during nanocatalysis. The presence of just hexacyanoferrate ions in the solution with the nanoparticles is found to increase the amount of distorted tetrahedral and distorted cubes present much more than during the reaction. The presence of only the thiosulfate ions does not seem to affect the size or shape distribution which might result from the capping ability of this anion and thus protects the nanoparticles.