The elastic modulus E of diamond is often set equal to 1/s11 equals 1050 GPa, which assumes that is does not vary much with orientation, and many authors use (upsilon) equals 0.2 as an appropriate average value of Poisson's ratio, which is incorrect. In fact, since the elastic constants of diamond are known with great accuracy, it is a straightforward matter to derive exact numbers for E and (upsilon) that take into consideration the stress direction, the intrinsic anisotropy, as well as the crystalline configuration. For CVD diamond deposits, we find that, in a first approximation, the Hershey-Kroner-Eshelby averaging procedures yields acceptable numbers, E equals 1143 GPa and (upsilon) equals 0.0691, which are quite compatible with available experimental evidence. Our measurements of the biaxial modulus, E' equals E(1 - (upsilon) ), made use of the bulge test method to characterize the elastic behavior of both microwave-power and hot-filament assisted CVD diamond films. High- quality deposits yield E' is congruent to 1180 GPa and E' is congruent to 1220 GPa for randomly orientated and (110) textured deposits, respectively: these results confirm that state-of-the-art deposits exhibit elastic properties that are in accord with the measured stiffnesses of natural single- crystal diamond. The residual hydrogen content strongly impacts the elastic behavior and appears to be responsible for the degradation of the modulus observed in this and previous work.© (1992) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.