The mechanism of 1,2-H migration process in photoexcited alkyl diazo esters was studied by femtosecond IR transient absorption spectroscopy and theoretical calculations. Two possible processes are discussed rearrangement in the diazo excited state (RIES) and the intermediacy of singlet carbene species. In chloroform solvent ultrafast IR data show a subpicosecond rise of-alkene species indicating the occurrence of the RIES process. Failure in carbene species detection is explained by their absence or very low quantum yield of formation.