The purpose of this paper is firstly a general analysis of piezoelectric elastic systems with constant volume, to which one can apply an equation of the kind f(L, Phi, q, T) = 0 with three independent variables. Subsequently, one deals with a more detailed analysis of elastic systems without piezoelectric effects with two independent variables in an equation of the kind f(L, Phi, T) = 0 analogous to the relation f(p, v, T) = 0 that is valid for fluid systems. In addition, a system with variable volume and, thus, depending on four independent state functions is synthetically examined. (C) 2001 Elsevier Science Ltd. All rights reserved.