In propylene carbonate as solvent (+( n-hex) 4NClO 4 0.1 M) the cryptate [222 Tl] + and the cation Tl + are electroducible. The reduction step is monoelectronic and slowed down for the cryptate compared to the uncomplexed cation. For the cryptate [222 Tl] +, by correlation of n.m.r. results on homogeneous ion exchange and combination with the electrochemical heterogeneous data, the standard redox potential of the couple [222 Tl] +/[222 Tl] 0 is estimated to be −0.73±0.02 V/SCE. This value is in agreement with the electrochemical results. The stability constant of [222 Tl] + is calculated by two independent procedures, from electrochemical results and from n.m.r. data. The results are concordant: log K s=9.0±0.3 at 25°C, in propylene carbonate (+( n-hex) 4NClO 4 0.1 M).