Summary Diazoles have attracted considerable attention for a long time owing to their potentially interesting chemical, biochemical, and medicinal properties. We have reported the synthesis and in vitro antibacterial activity of a new series of (N-substituted)-3-methyl-4-diazo-5-pyrazolecarboxamides 1a-n along with their quantitative structureactivity study. There was a trend towards a better Gram-negative activity with decreasing molecular refraction values of the substituents on the carboxamidic moiety and a better Gram-positive activity with increasing values of IR carbonyl shift. The compounds which displayed the broadest antibacterial spectrum were 3-methyl-4-diazo-5-pyrazolecarboxamides substituted with 2-pyridinyl and 3-isoxazolyl moieties, making evident the structural requirement of a heterocyclic substituent with aminic function located in the ortho position with respect to heteroatom(s). The introduction of new substituents with different lipophilic properties in place of the C(3)-methyl group such as cyano, methoxy, ethoxy, benzyloxy substituents have been examined followed by application of these materials to the cosmetic formulations. An emulsion utilizing the 2-pyridinyl compound at 0.2% and 0.4% has been prepared, and their specific antimicrobial activity has been evaluated. Preliminary antimicrobial challenge studies indicated the excellent preservative activity of this compound in cosmetic formulations.