A nucleophilic addition reaction between a hydrophilic hydrazide, the hydrazide of polyethylene glycol with a molecular weight of 3400, and a hydrophobic aldehyde, has been performed in a microemulsion and a lamellar liquid crystalline phase of a ternary non-ionic surfactant-water-oil system. As a reference the same reaction was performed in a two-phase (water-oil) system, without any surfactant. The phase behavior of the microemulsion system in the presence and in the absence of the reactants, and the temperature effects on the phase behavior were investigated. Addition of the hydrophilic reactant, the PEG dihydrazide, to the aqueous component, as well as addition of the hydrophobic aldehyde to the oil, resulted in a reduction of the temperature range of the microemulsion which was interpreted as being due to a change of the spontaneous curvature of the surfactant monolayer in the direction of increased bending towards water. The reaction kinetics were investigated by 1H-NMR. It was shown that the reactions performed in the oil-in-water microemulsion and the liquid crystalline phase were very rapid with almost all of the starting materials being consumed after a 20-min reaction time. The reaction performed in the two-phase system was very slow and after 6 h only 10% of the starting material had reacted.