We have investigated the effect of transition metal complexation on the swelling behavior in water of a new terpolymer gel made from N-tert-butylacrylamide (N-t-BAm) as the hydrophobic monomer, 2-acrylamido-2-methyl-1-propanesulfonic acid (AMPS) as the hydrophilic monomer, and N,N'-methylenebisacrylamide (BisAm) as the cross-linker. At a critical balance of hydrophilicity and hydrophobicity, the terpolymer exhibited a sharp volume transition at 15 degrees C. A shift in the transition temperature of the terpolymer was observed after complexation with trace metal ions. Electron paramagnetic resonance spectroscopy (EPR) suggested a tetrahedral structure for the complexes. We propose that the shift in volume transition temperature of the gel is due to subtle changes in the hydrophilic-hydrophobic balance resulting from the complexation of hydrophilic -SO3H groups of the gel with trace amounts of the metal ions.