Fischer-Tropsch catalysts (Fe/V oxides with ZnO and K,C03 as promoters) were exposed to CHCI,, thereby producing surface and bulk chlorides. The effect of this exposure on activity and selectivity was studied in a continuous recycle reactor at a total pressure of 10 bar (CO/H, in most experiments: ca 1: 1) in a temperature range between 200 and 343 "C. CHCI, was introduced in amounts of up to 1 x lo-' mol chlorine per g catalyst. The catalyst samples were characterized by internal surface area, pore-size distribution and adsorption capacities for CO, H, and CzH4. Prior to synthesis, the catalysts were reduced by H,. Catalyst exposure to CHC1, resulted in a decrease of activity and considerable changes in product distribution. Hydrogenation and isomerization of I-olefins were partly suppressed; the chain length of the products was slightly increased. Deactivation of the catalysts due to chlorine addition was partly reversible during operation, while olefin formation was not significantly altered with time-on-stream. The effect of chlorine on activity and selectivity is explained by dissociation of CO as the chain initiating step and CO insertion into a carbonhetal bond as a possible chain propagation step. Since adsorption capacity for H, decreases on the addition of chlorine, this may also contribute to lower activity and change in selectivity, compared to the unexposed catalyst.