The uniformity of wall thickness is one of the hardest specifications to fulfill for thick-walled polystyrene (PS) shells prepared by microencapsulation. To improve the wall uniformity of thick-walled PS shells, both density matching between the inner water phase (W1) and the oil phase (0), and that between the W1/O compound droplets and the outer water phase (W2) were intensively studied. The results showed that the centering of W1/O compound droplets, the location and movement of W1/O compound droplets in the W2 phase were significantly affected by the density matching level, which resulted in the variation in wall thickness. For PS shells with 850 lam inner diameter and 25 mu m wall thickness, when the density matching between the W1/O compound droplets and the W2 phase was in the range of -0.007 g cm(-3) to 0.001 g cm(-3), the wall uniformity of the PS shells became better. With the density differences between the W1 and the W2 phases ranging from -0.007 g cm(-3) to 0.007 g cm(-3), the wall uniformity of the PS shells can be further improved. By optimizing the density matching level among the three phases, the batch yields of PS shells with the wall thickness variation less than 5 gm increased from about 15% to above 84%. Compared with the wall uniformity, the sphericity of PS shells was much less sensitive to the density matching level. The relevant mechanisms of how the density matching affecting the wall uniformity and the sphericity of the PS shells were also discussed. (C) 2013 Elsevier B.V. All rights reserved.