The paper focuses on the effect of potassium content on the structure of alumina supported molybdenum sulphide catalysts and their performance in carbon monoxide hydrogenation at the conditions favouring olefin production. Molybdenum oxide is a major phase in the unpromoted calcined catalyst. The formation of mixed K-Mo, Mo-Al and K-Mo-Al oxides occurs in the calcined catalysts in the presence of relatively small amounts of potassium. The mixed potassium molybdenum oxides readily convert into mixed molybdenum sulphides after pre-treatment with H2S. The presence of potassium results in a signification increase in the concentration of basic sites in the sulphided catalyst, while their strength is not much affected by the promotion. The catalytic results suggest the presence of two types of sites associated to unpromoted molybdenum sulphide and K-Mo-S species. Higher olefin (and alcohol) production rates were observed on the mixed K-Mo sulphides, while carbon monoxide hydrogenation on upromoted molybdenum sulphide leads to methane. Molybdenum oxysulphide seems not to be relevant to olefin synthesis. (C) 2015 Elsevier B.V. All rights reserved.