Abstract: Based on diffraction theory, the determination of the gas-liquid specific interfacial areas in horizontal agitated vessels by laser transmission method is discussed and analysed. Better reliability and good accuracy are claimed with laser measurements than with normal light. Studies on specific inter-facial areas in horizontal vessels provide valuable suggestions for the design of polymerization vessels. Increasing the ratio of impeller diameter may prove to be a reasonable approach for enhancing gas-liquid contact. Furthermore, addition of small amounts of surface-active agents may bring about 10 to 25 times in specific interfacial areas.