Hydroformylation of olefinically unsaturated compounds

In a process for hydroformylating olefinically unsaturated compounds by means of rhodium/triphenylphosphine/carbonyl complexes formed in situ from rhodium acetate, triphenylphosphine and carbon monoxide, the improvement comprising:

• preceding the hydroformylation reaction with a starting phase wherein a mixture of the hydroformylation medium, triphenylphosphine and rhodium acetate is heated at from 90.degree. to 120.degree. C. under a CO/H.sub.2 pressure of from 5 to 20 bar and the acetic acid liberated is discharged from the reactor in gaseous form in a stream of CO and H.sub.2, until virtually no more acetic acid can be detected in this gaseous discharge, and thereafter passing the olefinically unsaturated compound into the reaction medium.