The hardness variation of amphoteric gel according to the surrounding solution conditions is quite unique. It hardens and softens reversibly regardless of its molecular network density. But this has been understood merely qualitatively. For the purpose of elucidation of the details of its behavior, we performed quantitative potential and hardness measurements on it. We observed the constant potential of amphoteric gels, similar to -60 mV, regardless of their swelling ratio and hardness. Such observations can be interpreted as the maintenance of the constant charge density of -COO- for any amphoteric gel, and they are further interpreted as intermolecular salt-linkage formation/disruption dominating the hardness of amphoteric gels. (C) 2004 Elsevier Inc. All rights reserved.