The viscoelastic properties during setting of commercial alginate impression materials vary considerably from one product to another. In order to obtain an acculate impression, it is important to understand these differences in viscoelastic properties. The purpose of this investigation is to develop the method of measuring viscoelastic properties during setting and to determine precise setting characteristic of commercial alginate impression materials. The materials used in this investigation were mixed according to the manufacturer's recommendation at a temperature of 23 +/- 0.5 degrees C and transferred after 60 sec from start of mixing to the testing instrument in an air cabinet at 32 +/- 0.5 degrees C and allowed to set. The mixing time for materials was 30 seconds. The apparatus used in this investigation is the oscillating rheometer modifiied by Inoue and Wilson (1977). The working times of impression materials procured at retail in alginate paste were shorter than those of alginate Impression materials in powder. Alginate, Jeltrate (regular), Xantalgin (normal) and Zelgan had long working time. In case of these materials, the variation of elastic modulus during the time of working was small markedly. All other materials used in this investigation, with the exception of Alginate, indicated a considerable increase torsional shear modulus after the setting time determined by the method of British standard.