Analysis of the deflection of a circular membrane under differential pressure (bulge test) is a well-known method of determining the elastic properties of thin films. However, analytical models always suffer from simplifying hypotheses. In this study we present a new approach, based on numerical modeling, to interpret pressure-deflection curves. By adjusting Young’s modulus and Poisson’s ratio in the simulation program, it is possible to reproduce the experimental curves faithfully. The method was successfully tested with two different materials (silicon and aluminium) with known elastic properties and was then used to determine biaxial Young’s moduli of CVD diamond thin films for three different microstructures. The values of E varied from 565 to 620 GPa (assuming a Poisson ratio of 0.1). Grain boundaries are thought to be responsible for the relatively low values of Young’s moduli. Uncertainties in E are relatively large (lo%-15%) because the method is highly sensitive to experimental parameters such as thickness or membrane diameter and to the initial residual stress state which is known only approximately.