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The reactivity of orthophthalaldehyde (OPA) with primary amines is a base for broad application of OPA: in the determination of amino acids, in medicine as a disinfection agent and in organic synthesis, therefore the detailed knowledge of the mechanism is of interest. The electrochemical reducibility of OPA was utilized for the investigation of its reactivity with eight aliphatic primary amines which could be separated into four types. Based on the electrochemical monitoring, the apparent reaction rates were evaluated. It was found that the reaction rate essentially depends on the substitution at the α-carbon atom of the amine. The quantum chemical computations revealed that the reaction rate well corresponds with the Hirshfeld and NBO atomic charges on the α-carbon atom. Because of the strong hydration of OPA in aqueous solutions, the reaction was performed both in aqueous solutions (pH 9–10) as well as in diethyl ether. For the identification of intermediates and products, NMR, MS and X-ray crystal structure analyses have been used. Besides the expected isoindolin-1-one main products, the phthalimide derivative as a side product was observed and a new type of the conjugate product has been found and identified. The plausible reaction pathway was proposed.