The propagation of acoustic Lamb waves in free standing chemical vapor deposited polycrystalline diamond has been studied using a laser ultrasonic technique. The influence of film morphology, quality, and thickness on the waves has been assessed. Acoustic waves with high velocities in the range 8700–12 200 ms-1 were observed; the lowest values were recorded for films with the lowest crystal quality and highest nondiamond content. High quality films with differing crystal textures or thickness show little variation. The influence of temperature on the dispersion characteristics of Lamb wave propagating in a 50 mm diam polycrystalline diamond wafer were also investigated. Little variation was apparent across the range studied (30–250 °C). Material parameters extracted from the dispersion chracteristics of the acoustic signal together with scanning electron microscopy studies suggested that void, microcrack, and grain boundary density most influences the propagation of low frequency Lamb waves in free standing CVD diamond films. © 2000 American Institute of Physics.