Abstract The hydroformylation of 1-hexene at 50 and 125°C and 300 psig CO:H 2 (1:1) using soluble and zeolite-supported rhodium species is reported. The presence of excess phosphine during homogeneous catalysis in shown to inhibit isomerization of 1-hexene and thus give high normal/branched aldehyde ratios for all levels of conversion. However, the absence of phosphine allows significant isomerization, causing the normal/branched ratio to vary with conversion. The activity of the immobilized catalysts is affected by the type and amount of phosphine. The homogeneous and heterogeneous catalysts can be poisoned by mercaptans, provided an excess of phosphine is not present. The data from the immobilized catalysts suggest that claims of intrazeolitic hydroformylation with rhodium-containing faujasites must be taken with caution.