We report here the first example of a stoichiometric hydroformylation using HMn(CO)5. Treatment of a hexane solution of 1,2-diphenyl-3,3-dimethylcyclopropene with HMn(CO)5 at 55°C gave after 5 h a 27% yield of aldehydes, 87% cis and 13% trans. The other major products were cis-(87%)-, and trans-(13%)-1,2-diphenyl-3,3-dimethylcyclopropane. Evidence for a radical intermediate is presented.