Hydroformylation of 1-hexene has been investigated over cobalt supported on active carbon catalyst (10 wt% Co/A.C.) in a slurry phase reactor under low pressure (3.0 Wa). Added alcohols, such as methanol and other lower alcohols significantly promoted the reaction to realize a low-pressure process for hydroformylation at 3.0 Wa and 403 K, markedly giving acetals and aldehydes. Effects of operating variables on the CO conversion, yields of products and stability of the catalysts have been discussed in detail. The addition of a small amount of Ru (0.5 wt%) to the Co catalyst remarkably shortened the induction period and increased the catalytic activity. Compared with the corresponding homogeneous catalyst, the supported cobalt catalyst was observed to have good stability in the continuous feed system, showing constant CO conversion and yield of oxygenates.