A series of aldehyde inhibitors with the general formula Ac-Phe-Val-Thr-X-CHO, where X = Lys, Arg, Phe, Tyr, p-nitro-L-phenylalanine (Nif), p-amino-L-phenylalanine (Amf), p-guanidine-L-phenylalanine (Gnf), pyridyl-L-alanine (Pal), was synthesized. The starting structure of this series based on our previous work on cathepsin G chromogenic substrates. The synthesis of all compounds was performed in solid phase applying Fmoc chemistry. We investigated the inhibitory potency of the obtained compounds against cathepsin G and bovine α-chymotrypsin and evaluated their dissociation constants (Ki). The studied peptides displayed different inhibition profiles and potency. As a result, a potent and selective inhibitor of cathepsin G with the sequence Ac-Phe-Val-Thr-Gnf-CHO, displaying Ki = 22 nM was obtained.