Ordered lattice structures, face-centred cubic (f.c.c.) and body-centred cubic (b.c.c.) lattices, are systematically analysed by the reflection spectrum measurements of monodispersed polystyrene latices in deionized solution. The nearest-neighbour interparticle distance (2D_exp) is determined from the wavelength at the reflection peaks. 2D_exp decreases with increasing latex concentration and is smaller than the calculated average values for the f.c.c. or b.c.c. distribution (2D₀). The results are consistent with a ‘two-state structure’ which has ordered and disordered regions. The f.c.c. and b.c.c. lattices are stable at high and low concentrations, respectively. The b.c.c. structure often appears at a high solution temperature and in the presence of a small amount of foreign salt. The size of the ordered region is estimated from the width of the reflection pattern and also from microscopic photographs. The number of crystal layers is estimated to be several hundred and depends on latex concentration and solution temperature.