Abstract A study has been made of the degree of flow exhibited by 15 different impression materials commonly used in prosthetic dentistry and the amount of flow which might be expected from these materials at various time intervals following the beginning of spatulation. The research substantiates previous work where it has been found that the degree of flow varies from thin materials which exhibited almost unlimited flow, to those which were comparatively stiff. Some materials demonstrated a critical decrease in flow at room temperature, thus giving a limited amount of working time, while in other materials the flow remained high. Two of the seven zinc oxide-eugenol pastes, the alginates and the plaster-containing materials demonstrated this marked decrease in flow at certain time intervals. It is apparent then that if a specific flow is desirable, the time between mixing and insertion of the tray is critical for certain materials. The rubber base materials were not markedly influenced. This marked reduction in flow, as exhibited by some of the materials, might be advantageous in reducing the possibility of distortion while the materials are setting in the mouth. Although some materials showed a definite correlation between the flow and setting time, the reduction in flow could not always be predicted by examination of the setting time.