The Hall coefficients ( R H ) and the electrical resistivities (ρ) of three beta-phase alloys CuZn, AgCd and AgZn, which undergo the order-disorder transformation, were measured from room temperature to about 615°C. Experiments showed: 1. R H of the ordered (β') phase connects continuously with that of the disordered (β) phase, at the transition temperature; 2. R H varies rather slowly in the region where ρ changes rapidly; 3. R H varies linearly in both β'- and β-phases and the temperature gradient of R H is greater in β-phase than in β'-phase. To explain these results, a gradual change of the Fermi surfaces in the vicinity of the transition temperature was proposed.