As previous evidence shows, the distribution of gas-solid flow traveling through identical parallel paths can he significantly nonuniform, often with harmful operating consequences in practice. A fundamental analytical and numerical study is performed on the distribution of gas-solid pneumatic flow passing through a "Y branch". While many steady-state gas-solid distribution solutions, including a uniform distribution, satisfy the governing equations, linear stability analysis indicates that the uniform distribution is stable the most likely solution of the system. Both 2-D (two-dimensional) and 3-D multiphase computational fluid dynamic simulations and stability analyses confirm the analytical conclusions. (C) 2010 American Institute of Chemical Engineers AIChE J, 56: 2039-2051,2010