Thiolation of methanol with H2S was studied on Al2O3, WS2/Al2O3, and the Cs-modified counterparts Cs/Al2O3, and Cs-WS2/Al2O3. On Cs-free catalysts, methanol reacts with similar rates to dimethyl ether and to methanethiol. Secondary steps yield dimethyl sulfide and trace amounts of dimethyl disulfide and methane. On Cs-containing catalysts, the dominating reaction is methanol thiolation. This drastic change in selectivity is related to the low strength and concentration of Lewis acid sites in the presence of Cs+. The active sites for condensation and thiolation of methanol are concluded to be Lewis or Bronsted acid-base pairs formed by dissociation of H2S on the Lewis sites. Thiolation and condensation follow Langmuir-Hinshelwood mechanisms with nucleophilic attack of SH or methoxy groups to adsorbed methanol as rate determining step. The formation of dimethyl ether on Cs+ Lewis acid sites is suppressed due to low coverage of methanol, whereas the rate of the nucleophilic attack of SH to methanethiol is enhanced. (C) 2016 Elsevier Inc. All rights reserved.