Elastic and thermoelastic properties of selected organic crystals: acenaphthene, trans-azobenzene, benzophenone, tolane, trans-stilbene, dibenzyl, diphenyl sulfone, 2,2 '-biphenol, urea, melamine, hexogen, succinimide, pentaerythritol, urotropine, malonic acid, dimethyl malonic acid, maleic acid, hippuric acid, aluminium acetylacetonate, iron acetylacetonate, and tetraphenyl silicon

Elastic and thermoelastic properties of the 21 title crystalline species ha ve been determined from ultrasonic resonant frequencies of plates and their shift upon variation of temperature, respectively. In addition, the tensor s of thermal expansion have been measured by the aid of a Fizeau interferom eter. An analysis of the elastic behaviour of organic crystals, including earlier elastic data of other organic compounds taken from the literature, reveale d common features of organic crystals and organic liquids in respect to the scalar invariants C=(c(11) + c(22) + c(33) + c(44) + c(55) + c(66) + c(12) + c(13) + c(23))/9 and d log C/dT, T temperature. especially in the case o f Van-der-Waals crystals. Contrary to ionic crystals the invariant C increa ses in these crystals with larger molecular volume. Crystals with strong hy drogen bonds like dicarboxylic or sulfonic acids behave rather like ionic c rystals. The influence of a small molecular elastic stiffness, as observed in plastic crystals, is clearly recognizable in the elastic properties. The impact of structural details on the invariant C and its logarithmic temper ature derivative is surprisingly small.