La3S4 is known to crystallize in a defect cubic Th3P4 structure. The effect of alkali metal ion doping in the lacunar La3-xS4 structure has been studied. Compounds with molecular formula La3-xAxS4 (A = Li, Na and K) with varying A/La ratios (0.1, 0.15, 0.2, 0.25 and 0.3) have been synthesized by gas-solid reaction method. The band gap increases with increase in dopant concentration. The optical properties show an increase in yellowness of the doped compositions when compared to the parent phase. © 2006 Elsevier Ltd. All rights reserved.