Abstract The room temperature structures of the two-layer Aurivillius phases Bi 2.5 Me 0.5Nb 2O 9 ( Me=Na, K) have been refined with the Rietveld method from powder neutron diffraction data ( λ=1.470 Å). They consist of (Bi 2O 2) 2+ layers interleaved with perovskite (Bi 0.5 Me 0.5Nb 2O 7) 2− ( Me=Na, K) slabs. The structures were refined in the orthorhombic space group A2 1 am, Z=4, and the unit cell parameters of the two oxides are a= 5.4937(3), b=5.4571(4), c=24.9169(14) Å and a=5.5005(8), b=5.4958(8), c=25.2524(16) Å, respectively. The orthorhombic distortion increases with decreasing Me + cation size in the perovskite layer (Bi/ Me) 2+ site and the lone pair electrons from the Bi 3+ cation are influencing the site distortion. This is in agreement with other two-layer Aurivillius phases and originates from bonding requirements depending on size and electronic environment.