The increase in short-range order in alpha brass (25% and 30% Zn) produced by thermal treatment or neutron irradiation is accompanied by a measurable decrease in lattice parameter (up to 0.02%). The equilibrium values of the lattice parameter decrease markedly with annealing temperature in the lower range of the measurements, i.e., in the vicinity of 125°C. The activation energies calculated from the kinetics of the lattice parameter changes are 40.7 kcal/mole for the 25% Zn alloy, and 39.5 kcal/mole for the 30% Zn alloy, in agreement with results of anelastic measurements and electrical resistivity studies. From thermodynamic data it is estimated that a decrease in lattice parameter of 0.01% corresponds to an increase of about 0.05 in the Bethe short range order parameter. © 1958 The American Institute of Physics.