The solubility and solubility product of erbium trifluoride in aqueous solution were measured using three different techniques. The more acceptable radiometric values for the solubility and solubility product are 1.98×10–5M and 3.5×10–18, respectively. The values for the thermodynamic functions, H
298o
,
G
298o
and
S
298o
for the dissolution of ErF3 and the stability constant for ErF2+ have also been measured.