La2NiO4 (+) (delta)-based materials (LN) with tetragonal K2NiF4-type structure exhibit high electronic and oxygen ion conductivity. In this work synthesis, phase relations and oxygen transport properties are reported for the system La2 - ySryNi1 - xMoxO4 (+) (delta) where 0.0 <= y <= 0.4 and 0.0 <= x <= 0.1. The solubility of Mo in LN is low, but becomes enhanced by Sr substitution on the A-site. La1.8Sr0.2Ni0.95Mo0.05O4 (+) (delta) (LSNM) bar shaped samples were subjected to electrical conductivity relaxation (ECR) for assessment of oxygen bulk diffusion (D-chem), oxygen surface exchange (k(chem)) and electronic conductivity. The p-type conductivity was 50 S.cm(-1) at 900 degrees C, which is lower than both LN and LSN (Sr = 0.2). On the other hand, k(chem), was enhanced by one order of magnitude compared with LN at all temperatures, while D-chem was lower at T > 700 degrees C and higher at T < 700 degrees C. (C) 2016 Elsevier B.V. All rights reserved.