First ever non-deformylating transdiazotization of acylacetaldehydes was achieved: the reactions of 2-azido-1-ethylpyridinium tetrafluoroborate (4) with acylacetaldehydes 3 proceeded partially without deformylation to yield 16 new alpha-diazo-beta-oxoaldehydes 1 along with diazomethyl ketones 2, especially in the presence of NaOAc (Scheme 1, Tables 1 and 2). The product distribution was substituent-dependent and could be correlated quantitatively. This new diazotization reaction appears as an alternative, direct, and more general method for the synthesis of these diazooxoaldehydes. alpha-Oxocycloalkanecarbaldehydes 5 gave only traces (if any) of alpha-diazocycloalkanones 7, and rearrangement products 6 were isolated (Scheme 2). Mechanisms of the reactions are discussed (Schemes 4 and 5).