The first part of this paper deals with the interaction of first and second nozzle pressures in air-jet spinning of microdenier polyester/cotton blended yams. The results show that the first and second nozzle pressures interact with each other to determine yarn strength. This is because both the number of wrapper fibers and the length of wrappings formed at a particular first nozzle pressure depend on the level of the second nozzle pressure and vice versa. There is an optimum number of wrapper fibers and wrapping lengths that yields the maximum yarn strength, and this optimum level can be obtained at several different nozzle pressure combinations because first and second nozzle pressures interact with each other. However, it would be advantageous to use the lowest of these nozzle pressure combinations because of significant savings in energy costs. The second part of this paper investigates the effect of yarn delivery speed, first nozzle pressure, and blend ratio on the hairiness profile of microdenier polyester/cotton blended air-jet spun yams. The results show that yarn hairiness increases with increased yarn delivery speed and first nozzle pressure, but decreases with increasing amounts of polyester in the yarn. Also, the three factors (yarn delivery speed, first nozzle pressure, and blend ratio) interact with one another in determining yarn hairiness.The structure and properties of air-jet spun yarns are determined by the fiber properties and process parameters used to produce the yarns. The important process parameters that affect the properties of air-jet spun yams are first nozzle pressure, second nozzle pressure. total draft, interjet distance, production speed, and thread tension draft 18 1. Various authors [ I , 4, 5, 8, 9. 11 ] have investigated the effects of these process parameters on the properties of air-jet spun yarns using different fibers and yarn counts, but their interactions have not been investigated thoroughly.When two process parameters interact with each . other, the effect of one process parameter on yarn properties depends on the level of the other process parameter. For example, Chasmawala et al. [41 [ showed that first nozzle pressure interacts with the main draft to determine the strength of 100% polyester air-jet spun yams. When the main draft is set at 20, yam strength ' increases with increasing first nozzle pressures. However, when the main draft is increased to 50, yam strength does not change with increasing first nozzle pressure. In earlier work, we ( Rajamanickam et ul. I I I I),investigated the interactions of first nozzle pressure, yam count, and blend ratio, and of second nozzle pressure. yarn count, and blend ratio. But the interaction of first and second nozzle pressures has not been investigated, and the first part of this study fills that need.With the advent of high speed weaving machinery, controlling yarn hairiness has become extremely important because the performance of these high speed looms is critically affected by undesirable hairiness. Thus, hairiness is...