The expansion coefficients of hexagonal (α) zirconium have been calculated from the lattice parameters of both low and high hafnium alloys in the range 0° to 600°C. It is found that the coefficients are straight-line functions of the temperature and that the effect of hafnium is to depress the mean coefficients by about 1 pct at room temperature and 9 pct at 600°C. The cubic (β) form was stabilized so that its coefficient could be determined from 400° to 900°C.