AbstractEquations are derived for stress–strain curves for a hypothetical model substance containing a large number of like systems, each assumed to be in equilibrium between two states having different arrangements of the atoms. With this model one can study the dependence of the initial elastic modulus, the limiting strain (for infinite stress), and the over‐all shape of the stress–strain curve on characteristics of the rearranging systems—their number, the initial energy difference between the two states, the shift of atomic positions, etc. Introduction of an assumed proportionality between the shift per rearrangement and the square of the relative length of the sample (in the direction of the shift) leads to stress–strain curves similar to those determined experimentally for natural rubber and other rubberlike