Monte Carlo simulations of binary Lennard-Jones crystals with mole fraction x = 0.5 are performed at constant temperature and pressure. In our symmetric model, the interactions between equal particles are the same (epsilon(DD) = epsilon(LL) and sigma(DD) = sigma(LL)). The interaction between D and L particles is changed by epsilon(DL) = e epsilon(DD) and sigma(DL) = s sigma(DD). The parameters e and s represent interaction strength and distance, respectively, as deviations from the Lorentz-Berthelot mixing rules. Gibbs energies were calculated to determine the stable crystal structure as a function of e and s, separately. This resulted in demixing for e < 0.93 and solid solutions for e > 1, with a weak transition to a substitutionally ordered fee at e > 1.8. Variation of s resulted in various crystal structures: a CsCl structure for 0.8 < s less than or equal to 0.95, NaCl structure for 0.6 less than or equal to s less than or equal to 0.8, ZnS structures with gradual transition to a double fee structure for s < 0.6. We conclude that small variations in the interactions between unlike particles in a mixture suffice to change the crystal structure completely. (C) 1997 American Institute of Physics.