The aim of this work was to develop a method for determining the solute lattice contraction coefficient of boron and phosphorus in silicon. An expression was derived which relates the curvature of silicon slices, one surface of which has been diffused with boron or phosphorus, to the amount of diffused impurity. The expression can be written as t 2 ϱ =6β ∫ 0 ∞ C(y) dy where ρ is the radius of curvature, β is the lattice contraction coefficient, t is the thickness of the slice and C( y) is the impurity concentration as a function of depth of penetration. The slice curvatures were measured by an X-ray technique and the amounts of diffused impurity were determined from sheet resistance and junction depth measurements. A plot of t 2/ ϱ vs. the amount of diffused impurity gave a straight line from which the solute lattice contraction coefficient, β, was derived. The relationship was linear over the complete range of experimental values. Stress relief mechanisms, such as dislocations or precipitates, were not observed even at the highest concentrations. From the plots, the solute lattice contraction coefficient was calculated to be 5·2 × 10 −24 cm 3 atom −1 for boron and 1·0 × 10 −24 cm 3 atom −1 for phosphorus. These values are critically compared with other values, derived by different techniques, reported in the literature.