With rhodiumcarbonyl/t-phosphane complex catalyst systems it is possible to hydroformylate cycloheptatriene at temperatures between 90 and 135 °C to diformylcycloheptanes mainly. The amount of triformylcycloheptanes does not exceed 10%, even under the most favourable reaction conditions. With cobalt carbonyl, which is surpringly active in the hydroformylation of cycloheptatriene, and also with rhodium carbonyl without the tertiary phosphane cocatalyst, only monoformylcycloheptane (< 1% diformylcycloheptanes) is obtained.